Well, I don't feel like typing anything up, seeing as threads/wiki's already exist, so read to your hearts content:
Edit: I can't find the Warhawk thread. I remember there was one, but I can't seem to find it. Anyone who finds it/makes it, feel free to post it here. I'll post the wikis too, just so Warhawk gets some information.
Both are blatant Battlefield clones. However the 3rd person camera of Warhawk makes my eyes bleed after awhile, and vehicles are horribly unbalanced (from what I played in an EB games). Frontlines is 1st person, and is an uncanny clone to BF2, and has a few problems with balancing the grenade launcher on the Assualt class (it needs nerf), and they need to beef the helicopter rockets (it took me like 4 to kill one stationary guy), other than that, no problems (from the demos I played).
Warhawk has 32 player max, Frontlines has 50 players.
Other than that, I don't know enough (outside of my 10 minute spree in EB games) about Warhawk to judge. However from what I've seen, it's nothing amazing.
You can download Warhawk from PSN, and the Multiplayer and singleplayer demos of Frontlines from XBL Marketplace.
As for which game is better, I see two clear advantages in Frontlines FoW right away: 50 players max. which is superior to the 32 from WarHawk as well as Frontlines FoW having single player while WarHawk lacks that.
I see, however to be honest I could care less about single player as the multiplayer is what really makes these games worthwhile.
But at the same time single player isn't a total waste as it helps you get familiarized with the gameplay a bit. Then you can apply what you learned to multiplayer.
What I'm saying though is that what you say would be true, if this was BF2:MC.
However this game actually has a compelling (albeit bleak and depressing) storyline, decent AI (they use cover, flank, ext), voice acting that WASN'T performed by gutter-trash down on their luck voice actors, and moderate scripting in the singleplayer game.
It isn't a full blown experience, but it's an actual game, unlike BF2:MC or Star Wars Battlefront that was, as you said, just a training ground for the multiplayer component.