I'd say that if you are going to spend a good chunk of time talking to somebody that has an AT&T plan, it might be prudent to go with AT&T since you will get unlimited "mobile to mobile" minutes, which means any calls to other AT&T customers will not be deducted from your monthly minute allotment. Also AT&T has "rollover", which means that unused minutes get put in a reserve in case you go over your monthly allotment.
As far as phones go, I'm using an 8925 Tilt
(Link). It's a little higher than your limit ($150 for a refurb w/a 2yr contract), but it's basically a mini PC.