|The Levi 501 (left) vs. the Levi 505 (right)|
First off, the flys of the Levi 501 and the Levi 505 are not the same. The Levi 501's have a button fly while the Levi 505's have a zipper fly.
Lastly, the fit of the Levi 501 vs 505 is slightly different. The Levi 501's taper down a bit to meet your legs, whereas the Levi 505's are a straight cut jeans with no tapering.
Your decision to buy the Levi 501 or the 505 comes down to your preference of zipper vs. button fly and whether you like your jeans to taper a bit. Either choice will be fine for most people. Levi's offers a great quality jean at an affordable price.