We use both Force 1's and Force 3's in the team, and to be honest there isn't that much difference between them. The Force 3's have marginally more padding in them so would be a must for those with current/past knee injuries. However, if you were concerned about finances, you could buy the Force 1's for £20 and get a pair of TSG knee gaskets from toddtwist, also £20:
http://cgi.ebay.co.uk/TSG-Skateboard-Kn ... 3ef40d9adbThe postage price for the pads and gaskets is quoted as £7.00, but he's usually fairly good about combining costs for multiple orders.
Don't worry about buying the next size up of kneepads if you go for the gasket option: TSG's have such long straps that they should easily take a pair of gaskets under your normal size pads.
Cheers
Trix xx