I don't have any experience with the Hi-Power, but I can give you some stats, and some opinions.
Technical Specs:
1. P99 is 10 oz. lighter than the Hi-Power (25 oz. vs. 35 oz., meaning the HP weighs 40% more!).
2. P99 is more compact, making is easier for concealed carry (7" vs 7.75" length, 1.2" vs 1.375" width).
3. P99 has a shorter barrel (4" vs. 4.75", probably means slightly reduced muzzle velocity with the P99).
4. P99 is a newer, more modern design, with excellent ergonomics (probably the best grip in the business).
5. P99 lacks an external safety "switch." Although it has many internal safety features, it can ALWAYS be fired by pulling the trigger, when the chamber is loaded.
6. P99 is more expensive ($799 list vs. about $670? list)
Opinions:
1. P99 is the best looking of the new polymer frame pistols. I don't think the HP has very good aesthetic qualities.
2. Smith&Wesson SW99 is a better value than the P99. But I personally like the look, fit and finish of the P99 better (they are basically the same pistol).
3. P99 trigger is 5+ lbs in single-action...too heavy for comfortable target shooting. It also has, what seems to me, excessively long trigger-travel, in both single and double action modes (this may be intentional, in order to "make up" for a lack of an external safety). I'm getting the trigger on my P99 in 9mm worked on right now to both smoothen and lighten the trigger (both single and double-action), to under 4lbs in single action mode.
4. Lack of an external hammer takes a little getting-used-to. Cocking the "hammer" after you "decock" it, is awkward, requiring you to pull back on the slide about 3/8". This is a bit cumbersome, and it takes some practice before you can reliably do this without pulling too much and accidentally ejecting the round in the chamber. Its probably best to fire the 1st round in double-action mode and forget about cocking it, after you use the decocker.
5. The Walther name is more prestigious than Browning. (In my mind, whenever I think of Browning, I think of good sporting O/U shotguns. "Walther" says "Top quality pistols.")
I would definitely pick the P99 over the HP. However, if you have a lot of experience with the HP already, I'd probably recommend that over the P99, since you'll be much more familiar with its handling characteristics. But variety is always nice!
[This message has been edited by Sambonator (edited September 11, 1999).]