If it's just instrumental & pitch isn't an issue "repitch" is your best option. repitch just adjusts the samplerate to play slower or faster so there isn't any artifacts. This is fine unless your doing extreme tempo changes. It's the same as beat matching 2 vinyl albums. Otherwise complex is the best for a full mix. The other modes are made for independent instrument tracks.

I am all about repitch mode for playing out with. It's the only mode that will really preserve the sound (with the exception of key, of course).
I personally can't stand complex mode as I've noticed that it not only sounds metallic and mp3-like, but also kills alot of the transients and punch in a tune. I have used it for doing remixes or re-edits of, say faster tracks that I want to do a slower tempo rmx of, but want to keep the original key intact. I will then strap some sort of a transient plugin (like Waves TransX, or Flux's Bitter Sweat) just to try and bring some of the kick back that complex mode has lost.
For some music Beats mode is fine and actually sounds good and punchy. But any material with sustained sounds (ecspecially longer bass tones) you will immediately hear that Ableton-Warble.
So repitch is your best bet ,sound quality-wise, but as Eric said it IS the most like vinyl beatmatching causing you to have to be very aware of clashing keys in different tracks that you're trying to blend together.
That my 2-cents.
And in addition to using re-pitch mode, I can't stress enough, how good the psp vintage warmer sounds if you throw it on abletons master output. It really does make ableton sound the closest to actual vinyl.