Are you going to carry the laptop a lot ?
Do you care about weight, size ?
If the anwsers are Yes, then, you should go with a 15.4" laptop.
I tell you this because I hate 17". I find them too big, too heavy, bulky and not as easy to carry as a 15.4" laptop.
But if you really want to with a laptop of that size, then, this HP seems good.
The differencebetween the T5300 and the T5500 isn't noticable. Maybe if you do a lot video encoding or file compression tasks you'll notice a little difference, but it's not like comparing a T2300 of the first generation of Core Duo with a T7500 of the latest Santa-Rosa platform where the performance gain overpasses the 250 %.
You should also take a look to the future Dell Vostro ( in replacement of the actual INspiron line ). According to Wikipedia, Vostro laptop are coming on the market on June 26th.
I was planning on getting this laptop
However, I read in some reviews the "better" laptop has a T5500 processor and not a T5300. Is the difference that major that it's worth the upgrade?