Personally, now, I think both are fine.
Blockland is fun because you can make addons, do a lot of other things ROBLOX can't do, there's a -slightly- better community, and it's a one time purchase, no extras like Builder's Club or things like that.
However, ROBLOX has a lot better graphics, has -SOME- good admins, and a nice coding language that anyone can learn. There are also cool contests that happen from time to time (remember the summer voting event?), like the Easter Egg hunt.
So, personally, it's a tie.