Never used AlterEgo ... but the sticker-price at BBTS is almost always cheaper than Sideshow, and its usually cheaper including shipping. Joker 2.0 is $15 cheaper with shipping than SS. The TDKR Batman is $10 cheaper at BBTS with shipping.
Exclusives are sometimes the exception, though. The BB Batman/Wayne exclusive is the only time I used SS instead of BBTS. As I understand it, BBTS had to buy their stock through SS like we did -- and they had to add profit margin into the price.
If you do some weird calculation with reward points, it may break even. I've never found reward points to be rewarding enough to bother. If you live in CA and get charged tax, that may throw it off, too.
SnakeDoc