I was going to be buying some single effects soon (delay, wah and chorus probably.) Now I am considering getting the PodXT Live footboard. I have a few questions about it though.

Do you have to use the amp models?

How do the amp models interact and sound with a tube amp?

How are the effects?

Is this better than the BOSS GT-8?

This is going to cost me at least $350 so I want something that will last and will have enough of what I need.
Buy the single pedals...do yourself a favor.
The POD XT live is cool for home practicing and messing around, but not for true live use. Neither is the Boss, its worse.
IMO, the POD is best used as a digital preamp thru tube power. This is where it shines tonally for live use. Most people just plug it into their amp's input and use it with the amp models and speaker / mic modelling enabled. IMO, that's the worse possible way. I've used my Guitar Port in this manner, and it utterly sounds like crap.

The best is to use the amp models, but disable the speaker / mic modelling. And ideally, the POD should be connected to the amp via the Effects Return. Essentially, you're bypassing all the amp's preamp controls and using only its power amp, using the POD as as preamp. I've used my Guitar Port like this, and it's mind-blowing what you can acheive tonally. And with the other model packs available, it makes it an excellent tool that's as expandable as it is versatile.

If you want to just use the POD as an effects board, you can disable the amp and speaker models, and use it only for its effects. It works quite well in this application too, although like demea said, I'd go for stomps instead.

As for the GT-8 - it's an amazing processor, and IMO, sounds even better than the POD. But it's for a learning curve steeper than Everest a convoluted, counter-intuitive interface, and shìtloads of options that even the most experienced players would never use.
Guitars: Custom Lado Earth 2000-3, Custom ESP Explorer, BC Rich KKV, Gibson LP Studio, Greco SG, El Degas Stratocaster, Agile AL-3000, LTD EX-351

Rig:Marshall JVM410H + Marshall 1960A, Boss Noise Suppressor
The best thing about the POD is the amp modelling, and if you are running it through a regular amp, it's pretty much pointless. The effects it has is just a bonus for modellers ...

Id say go with the stompboxes unless you are going to get an Atomic amp or Tech21 PE60.