I find the games on the WMS platform to have better graphics and sound and be slightly more engaging than IGT games. The bigger advantage of the IGT is that you can put more than 20 games of your choice on the machine. The WMS has a few gamechest sets (and other than the gamechest 14 which you are mentioning, all of the other gamechests are expensive $500+), after that expect to pay on average $200 per single game (and they cannot co-exist with the gamechest, in other words, you cannot add games to the gamechest, it's either the gamechest, or the single game ). With the IGT G20 (with show bios), you can add upwards of 60 or more games of your choice (with a few exceptions of some games that do not co-exist in a multi-game environment). You will need an IGT eKey 7 (or equivalent) that costs $400 or so in order to re-setup the game after a battery change, or if you add a game to the machine. With wms you will need a clear card which costs around $20 to $30.
These platform decision should come down to the games you want to play. WMS has some franchises that people enjoy a lot like bierhaus, jackpot party, monopoly, Lord of the Rings, to name a few. Since you are getting a BB2e cabinet with a bb3 mpu, you could also just buy a bb2 mpu by itself (usually under $300 now) and have access to the bb2 library of games as well. You cannot play bb2 games on a machine with a bb3 mpu, you will need to get the bb2 mpu.
IGT has a LOT of games for their video platform, along with poker games, blackjack, etc that can co-exist with slot games, so a fully loaded machine provides with a lot of different game play types, which is not something that is possible in the WMS platform (you can get poker games for it, but then you only have poker games). Also you will find the cost of IGT games to be about 1/3 or more less than what an average bb3 or bb2 game costs.
If you are going for a G20, make sure you get one with an MLD screen, makes it more flexible and enjoyable.