From my prospective, Both offer a good deal.
With Fictionwise, if you buy quiet regularly, the Club membership will pay for itself within a short while as I've found that the Club prices are usually a little cheaper then the BoB Prices. It does however require you to be willing to pay for the membership upfront tie up some of you money in Micropay dollars.
With BookonBoard, While you don't have the upfront cost of the membership, the discounted prices are a little higher, and you're Money is similarly tied up in Reward Dollars. The advantage is that Prices are lower then FW's non member prices.
If you buy eBooks only occasional, then BoB definitely offers a better deal.
However in my own case I've I got the Buywise membership for Free* (100% Rebate) and got a free book to boot, so better then free actually. The additional discounts is just gravy.
Also, while BoB usually follows when Fictionwise offers a sale In some cases, like the Chambers Promotion, they did not, and so I went with Fictionwise.
Basicly you just have to consider your own Buying habits and decide what is best for you.