You should buy the complete editions of both. They're both cheap ($20 total for both plus expansions). But if your looking at buying a specific one I would recommend HoI 2. They are both similar but I've always found HoI 2 to be the funnest.

HoI 3 is a good game. Its cheap ($40 AUD on Steam). Its imrpoved on everything from HoI 2 and added more. AI which will look after every part of the game (E.G: Playing as Britain you can focus solely on the Italian front in '43 while the AI looks after everything else). Its a great game worth its money. The only problems I've had with it are some minor ones (Can't make your own theatres...) and that the AI needs a LOT of improving to bring it to the level of HoI 2 or Victoria, but it still completely beats ETW...

HoI 2 with Doomsday and Armageddon is my recommendation.