Special Forces: More expensive, but big. 8(?) maps, with loads of extra features like grappling hooks, night vision, cool armies like SAS and Spetznas, new vehicles like ATVs, jet-skis and more. The night maps are a very different style of play, take some getting used to but really mix the game up a lot. The other SF maps include some real gems, like Warlord (which is THE king of close-quarters-combat, Karkand is a little girly map in comparison) and Ghost Town. SF puts an emphasis on infantry combat, and the importance of infantry squads working TOGETHER to make a difference.
Euro-Forces: A very mixed bag. Great Wall is a good map, with an interesting mix of lots of vehicles and some very close combat for infantry. The only EF map that suits ground-pounders, but it is very good for them. Operation Smokescreen is also quite good, allows for large-scale pitched battles between lots of tanks, with air support. Tabara Quarry is the weakest of the bunch, as the layout of the map means that it's far too easily dominated by good pilots. Overall, EF is... okay. Nothing special. Get it if the $10 means little to you and you want a bit of variety. The European equipment is quite cool (they have great medic and engineer weapons), but the voices are awful...
Armoured Fury: I really liked this pack. Obviously very focussed on vehicle combat, but unlike EF's vehicle-heavy maps there are less problems from aircraft, because there is a LOT more AA available, and the jets are slower. Each of the maps has its own flavour and style, and they're all pretty unique. The new jets and scout helo's are great fun, although you have to use them more intelligently than the vanilla jets or the aforementioned AA is going to ruin your day. Overall, the AF maps are good quality, well balanced, and offer new styles of play.
So... if I were you, I'd start with SF. It's great. After a while, pick up AF. If, after that, you still have a want for yet more variety, get EF, but bear in mind it's definitely the weakest of the lot. Hope that helps!