For a brief period in February 2015, I collected and owned all the Celestron Schmidt-Cassegrains.  I say "brief" because as soon as word got out, I started getting phone calls.  "Ed, you don't need all those - will you sell me your Cxxx?"  With people gnawing at my collection, it's been hard to keep the group together, although at this writing, the collection has managed to stay together.  Also, for you eagle-eyed observers out there, I am aware that these aren't all the SCTs that Celestron ever produced - there were early, blue-and-white Schmidt-Cassegrains that are very rare and collectible today.  I don't count those.  You have to draw the line somewhere, and I draw at around 1972, with the introduction of the C8, which started the current series.  I also don't count the various smaller Maksutovs like the C90.

Celestron C5

Celestron introduced the C5 as a smaller, lighter alternative to the C8, and it's been in the lineup ever since.  It's gone through several incarnations, from orange tube, to white tube, to black tube, and (as of this writing) back to orange tube.  Most of these have good optics and are acceptable for general viewing where portability is a concern.  Celestron's problem in the 1970s and 1980s was that it didn't cost much (if any) less to make a C5 than a C8, but for marketing reasons, they had to keep the price lower than a C8.  By the early 1980s, it was possible to trick out a C5 that cost more than a C8.  Offshore manufacturing has changed this, of course, and the new C5s can be had at attractive prices.  If you're looking for one, take your pick, as all iterations have their fans.  Some, for example, are staunch fans of the white C5s.  Me?  Much as I like the California-built scopes,  I have to admit the new inexpensive Chinese orange-tubed models have been really good lately.

The scope is small enough to fit on a CG-4 class mount with no problems.  Even a cheap alt-az mount like a Vixen Porta will work, and I've used mine in that configuration at star parties.  The C5 is also a good travel/eclipse scope, and a good alternative to a Questar, for those who don't have or can't afford those jewel-like creations from New Hope.

Celestron C6

I am old enough that I still view the C11 as the "new scope."  Silly, I know, because the C9.25 and the C6 are much newer.  Celestron introduced the C11 in the early 1980s, to fill the gap between the C8 and the massive C14.  We're solidly in big scope territory here, but if you've already got a C8 or a C9.25 and want to move up, I've got good news for you - the C11 is not that bad, size-wise.  If you can handle a C9.25, you can handle a C11.  At around 29 lbs, it's much heavier than a C9.25, but because it retains the aspect ratio of the C8 and the others, it's not much longer or bulkier.  I have no trouble mounting and dismounting it from my Losmandy G11.  And that's the bad news - unlike the C9.25, you will need a CGEM/G11/Atlas class mount for sure.  One good piece of news - the C11 will balance on a G11 mount with one 21 lb counterweight.

The 2800 mm focal length is really starting to box you in, but the C11 is a new favorite amongst lunar and planetary imagers.  In fact, the moon is my favorite target with the C11 - go figure.  The focal length is just right with my eyepieces.  I just completed the Astronomical League Lunar observing program, and the C11 was a big help.  The only issue is all the dang light it collects.  Your night vision is ruined for the rest of the night.

Celestron C14

A classic big scope.  Unlike the leap from the C9.25 to the C11, which is not that bad, there is a huge jump from the C11 to the C14.  Put another way, if the C9.25 to C11 gap didn't bother you and you get cocky (like yours truly) you are in for a surprise.  The extra 3" of aperture translates into ~60% more weight, and ~70% more internal volume.  And forget about Celestron's ads, which claim a 45 lb weight for the OTA.  My newer, triangle-back version tips the scales at 52.5 lbs bare.  Add a finder, big visual back, high-quality diagonal, and a big eyepiece like the 31 mm Nagler, and you can easily get up near 57 - 58 lbs.  A Losmandy G11 has a claimed weight limit of 60 lbs, so you're starting to outgrow that one too.  A CGE, Paramount, or one of the big A-P mounts is preferable.  

In the early years, Celestron took pains to advertise the C14 as a "one man" "transportable" scope.  They showed pictures of dainty models using it (on a fork mount, no less.)  They've backed off this claim in recent years.  I am able - just barely, and not without significant psychological and sweat equity - to put it on my G11 mount, but after doing it a couple of times, I have to conclude that this is a two-man scope.  At f/11, it has a longer aspect ratio than the others, which doesn't help when you're moving it around.  Physically, it more closely resembles the C9.25, than the C8.  

The C14 not a daily driver.  Its 3910 mm focal length limits what you usually look at, and if you don't have it on a permanent setup and use it three or four times a year, I think you are doing well.  In terms of light throughput, it resembles a good 12.5" Newtonian, but because of its ultra-long focal length, it feels totally different.  Imagine using your Orion XT12 with a 2.5" barlow permanently attached, and you start to get the picture.  The central star in the Ring becomes a real possibility, and the Dumbbell starts to take on that "big scope" football-like appearance.  The Virgo Cluster becomes a Virgo Clutter (that's supposed to be a compliment.)

The C14 gives lie to the notion that all SCTs are boring.  Everyone loves the C14.  Don't believe me?  If you get one, call your friends and tell them you have one.  They will come over to see it.

If you could have only one:

Get the C8. 

I know, I know,  I said above that the C9.25 is my favorite.  And you certainly won't go wrong with a C9.25.  But if I were starting out today and wanted an SCT, I'd get an 8" from Meade or Celestron.  Why?  It's still the best tradeoff for light gathering, portability, and cost.  Any gains earned by the C9.25 are negated by its heavier weight and the extra cost needed to buy a larger mount.  There's a reason the 8" was the first model introduced.  C8s and CG-5s are cheap and plentiful on the used market (good clean OTAs often sell for less than $400.)  Getting a C8 is like buying a Camry - it's not the most exciting thing in the world, but you know what you're going to get, and there aren't going to be any surprises.

If you can own two: