Houndoom
has a great learnset including: Inferno, Howl, Crunch, Flame charge, beat up, and good stuff like that. it has a horrible defense stat with 50, sadly. but you can teach it special attacking moves to protect it's defense. maybe give it flash fire.
Rapidash
had a great speed and attack but has a dumb stat of hp with 65. its attacks include: megahorn, (as you said) bounce, flame wheel, solar beam, fire blast, and stuff like that. it evolves at 40 giving you more time to train the EVs.
well in my opinion, I would choose Rapidash because of the stats. and in lvl 40s you will be at gym #6 which is steel. which if you didn't know fire is good against steel. you will also encounter a ice type gym leader for #7. I counted the platinum elite four and champion types to see that fire is good against 7 of the elite four's Pokemon. all is yanmega, scizor, vespiquen, heracross, bronzong, lucario, and roserade.
