Having purchased all the POD Studios, I am in good position to tell you which one will suite you the best. Essentially they all do the same thing, but as you move up the range you get more flexibility, additional functions and inputs/outputs. I think the easiest way to help you make up your mind, is to tell you what I use each of them for, so you will be able to see which would be the best match.
Line 6 POD Studio GX
So you are JUST a guitarist or Bassist who occasionally sings and all you want to do is get a cool riff or bass line, vocal into your recording software Then the GX is most likely for you, if you want something portable that you can bring to the rehearsal studio to get a quick demo from a live practice, then the GX will do that too, I have also taken the GX to the recording studio and used the line out to record straight into the desk. It has a single guitar/bass input and a single line/headphone output, comes with the very same POD Farm software that is bigger brothers come with and exactly the same near zero latency and nearly noiseless recording.
Line 6 POD Studio UX1
The UX1 is probably my favourite out of the bunch because you you still get a relatively portable device with a bunch of inputs and outputs. This is the one which stays attached to my Laptop and also what I use to do small acoustic gigs with! Yes I did just say that. Because you have an XLR input and 1/4 inch guitar/bass input you can output both into the mix, so I have a bunch of pre-sets made for all the songs in my set list and plug the left and right output straight into the PA. It works as a fantastic preamp and gives me total control over the sound. It also works better that the GX for recording acoustic guitar for the same reason. I usually take a line from the electro acoustic and then put a mic in front of the guitar and mix the 2 to get a really warm sound. The only thing I miss with the UX1, compared to the UX2, is the S/PDIF out, but this is just a small gripe because I like to hook up my UX2 to my home theatre system and use it as a monitor (when the wife is out!).
Line 6 POD Studio UX2
The UX2 is what I generally use when I need 2 condenser mics at the same time or if I am using an Acoustic with high output. The UX2 has 2 XLR inputs with optional phantom power (plus 2 assignable dials which show you the levels), you also have the choice between normal and pad input for the guitar/bass. Typically you would use PAD for a hot signal, like some of my Acoustics with piezo pick-ups. You could just turn down the output volume of the guitar, but you lose body in the recording so the PAD input is the preferred choice. You get an additional 2 knobs with the UX2 one gain for the additional XLR input and a separate volume control for the headphone output plus optional phantom power for the XLR inputs (check your condenser mic specifications to see if you need Phantom power or not). Essentially, the UX2 is a more professional, but less portable rig however, you can still only record 2 simultaneous tracks like the UX1.
Line 6 POD Studio KB37
The KB37 is a UX2 integrated with a midi controller keyboard. For those of you who know what a midi controller is and can see the benefit of having an integrated midi controller/USB audio interface read on! The obvious thing here is space saving, having said that, 37 keys isn't enough for most serious piano players, but if you are looking for something that can play your virtual instruments, is capable of controlling your favourite recording software using the assignable transport keys/Knobs and does everything that the UX2 also does, then this unit is for you. I am ambivalent about this unit really, I love the fact that it is a single unit, but I also like bigger keyboards with weighted keys. That said, mine has been used to trigger samples and add texture to live performances and remains attached to my mac mini and does a great job with GarageBand.