well, I own the PS3 Phat.
And though I feel that the phat looks much cooler with the glossy surface, its no big deal.
Given a choice today, when both the prices are same, I would go for the slimmer one, as it consumes lesser power and will be more reliable.
Also, if you carefully observe, the newer one doesnt have million holes like the older one, so I can leave it out, without fearing dust accumulation.
Regarding the performance, the PS2 purchased in the launch year 2000 will perform similar to the one made in 2009, simply because, the games should run on all the consoles equal and this is one of the biggest advantages of a console over a PC - The moment u enter the disc, you dont worry about frame rates, you just play!