Domain Reseller Options to start with are hard to compare. My suggestion is to first clearly set-up your individual criteria for selection - Then some basic questions need to be thought through such as volumes expected per type of domain etc, how much cross selling you expect to do, how much up selling you can assume given you are in web design business, what packages do you intend to offer your clients, whom are you planning to offer the services to - target clients, location, are they self service types etc.
Some pointers:
Customer service sucks across the board in our view
Avoid Smaller Players - Several reasons for this
Shortlist the top 3 then apply your custom criteria (Ours with years of experience is ResellerClub, Godaddy and CrazyDomains)
Look into taxation (Service Tax etc)
Avoid upfront fees or set-up costs - unless your business model/plan ensures you can recover it,
FINALLY - I would lean towards RC if I were you, however in our case we have a majority of our domain assets with CD for now (Rs 99 New Reg, 250 Transfer Ins and Rs 200 renewals) , we only use these for domains (no VPS, Hosting, SSL etc) - we do however have active sites which are RC based.