Both are capable cameras, so the better choice is often the one that fits you and your future lens plans best. Several people recommend handling both in a store, because ergonomics differ a lot and may matter more than small spec differences.

The D90’s main advantages are its more advanced body controls: separate command dials, a top LCD, and a larger grip that many people find more comfortable. It also offers slightly faster continuous shooting. Even though it’s older, it’s still regarded as a strong enthusiast-level DSLR.

The 500D is newer and lighter, which may appeal if portability matters.

A big long-term consideration is lenses. Once you buy into Canon or Nikon, most of your investment will likely be in lenses, so look at which system offers the lenses you want at prices you’re comfortable with. It can also be helpful if friends or family use the same brand, since you may be able to borrow lenses.

In short: choose the D90 if you want better handling and more direct controls; choose the 500D if you prioritize lighter weight. If possible, decide based on comfort in hand and the lens ecosystem you expect to build into.