Haven't tried Ubersmith, but I didn't have any good experience with WHMCS. Their support is not that good.
Ubersmith is overpriced in my opinion, unless you need the heavy customization for your business process.
If you want to move from WHMCS, HostBill and Blesta are great alternatives. We use Blesta ourselves and it has been delightful.
Never tried for Ubersmith, but it is quite costlier. WHMCS is also a better option for Ubersmith.
I think whmcs provide 2 versions of whmcs, simple and advance.