I have a 360g acrylic tank. It's 7' long and made of 3/4" acrylic. It doesn't show the slightest sign of bowing. I've only had water in it once, but prior to me owning it there was water in it for 6 years.
One thing to watch out for (and I saw this on RC somewhere recently) was an acrylic tank where the top bracing was not made from one piece, rather separate pieces (like a glass tank) and it let loose causing the front panel seams to start leaking. The top brace must be machined from a single piece, other than that it's all good
If you have the money for Starphire / low iron glass I'd go that way due to the whole scratching thing.