power rack. You can strengthen your abs and adjust your squat form (ie: arch the lower back, wider stance, push abs against belt, head up, better grip on bar, etc etc. Read anything by Dave Tate or the EFS boys).
The Smith Machine is just junk. Using that got me nowhere in terms of strength gains. I used it when i was doing HIT and bodybuilding. Complete effin waste of time.
The rack still has safety pins, so you're not going at it alone. Just train properly, bring a partner or three to spot you.
I think it's a complete mistake to go with the Smith. It's much heavier, and will be harder to move (in the event you do move). The power rack allows for so many options- just look at what you can customize over at EFS. Smith machines basically come in three kinds
A) lightened bar
B) 'heavy' bar (where you can feel the 45lbs of the bar)
C) P.O.S. Coathanger (either of the above).
Sorry man, but I trained Smith for so long, switched to dbell shoulder press for a change during one workout. I had no shoulder stability and blew my shoulder out (tore 3 of the four muscles around the rotator). I will never use that machine again.
Think about it in terms of breasts- would you rather check out a Rack, or a Smith? The name says it all... (i gots nothin')