Both games are indeed different, like it or not.
TF2 is based on red vs blu *not blue* to which you pick your side in each game. It has small to medium maps with no vehicles. There are a good amount of classes to pick from, and when you unlock weapons they can change to a different play style. The action is fast, the graphics are sexy, and the overall game is solid and fun. I bought the orange box right when it came out and have been active in TF2 ever since. I highly recommend buying the Orange Box, or TF2 alone.
BF:H is based on Nationals vs Royals to which you are stuck with the side you pick in every game unless you make a new hero. Map sizes go from small, medium, and large with some featuring vehicles. There are 3 basic classes to pick from, and when you lvl up you get ability points. They allow you to use unique skills for the class you pick. You can also rent weapons that allow you to change your play style. The action is moderate, the grahpics are funny, and the overall game is cartoony but fun. I have played BF:H for awhile now and I recommend everyone at least try said game as it is free to download and free to play. I'm loving my dual pistol commando. <3
I enjoy playing BOTH games because they ARE different. Fyi Valve does "free weekends" so you can try TF2 before you buy.
Also, no longer active, but you might want to join:
http://steamcommunity.com/groups/onrpgtf2
~Roh