Although Cable has a point about teamstacking the booster packs both offer a really refreshing experience in terms of guns, vehicles and maps.
-They both have really good map design (EF's Taraba is the only letdown for me)
AF has:
-littlebirds
-very unique bombers unlike anything you're used to in vanilla
-extremely unique map design, locations in North America rock (highway -> OP Road Rage, Pensyllvania farmland -> OP Harvest and Alaska -> Midnight Sun)
EF has:
-A new faction: the EU which means new skins and awesomely funny voice commands
-New guns! You might have the L96A1 and the P90 in vanilla but only EF players know the joy of the FAMAS (!), the HK21, the Benelli (!), the L85A2 and to a lesser extent the spec ops gun.
-New vehicles: Eurofighter (jet), Tiger (think Z-10 shape with the cannon of the MI-28 but crap handling) and two new tanks, the L2A6 and Challenger 2 that have the turret on the opposite side of your hud and are shaped differently compared to vanilla armor.
-Sweet maps (Great Wall -> it's the freaking great wall and you can fight on/around it in BF2, OP Smoke Screen -> ultimate vehicle whore map and yeah, um Taraba..)
GET THEM BOTH!