I have the older XT pro rackmounted version as some of the older members would tell you from my posts over the last few years i've had a love/hate relationship with Line 6 and my XTpro...i have it in my rack at the moment mainly for the programmable stereo effects loop,reverb and some stereo delay patches for which it isn't bad...as it's the older version and not the X3 i won't bore you with all the details unless you want me to elaborate futher with my experiences....
M13 has more connections with the same sounds, but is too big in my opinion. The M13 was prefered to the X3 in the mag FUZZ if I remember it correctly (for simplicity i think).
Well, I took the plunge and got the X3 Live. I read a bit about the M9 and M13 and decided to get the X3 for direct recording. Neither the M9 or M13 have amp simulation. And I am pretty sure that the Live will do any effect that the M13 though it's not as easy to use. I should have it on Tuesday or so. I will let you know what I think.
I own the Pod X3 Live, i love it, when it was new i kinda hated it, but i have like a year and few months with it and i still discovering new things, its the kind of units that you need a lot of time to get the really cool tones, with all the options and dual tone feature you can archieve a lot of great sounds.