Let's try to take an accurate list of pros and cons here.
ROBLOX
Pros:
- The gameplay is nice and smooth.
- Permanent brick physics can be used.
- A more advanced scripting system, capable of more complex functions.
- Lots of customization options, including the ability to add your own items.
- It's kid-friendly.
Cons:
- Not the best community.
- A lot of things cost money, although the game is (somewhat) free to play.
- Not the best administration, they could do a better job.
- The game can be "hacked" somewhat easily
- Sometimes, you get kicked from the game session for no apparent reason.
Blockland
Pros:
- Very smooth and fluid gameplay.
- An easy event system, as well as a well-rounded scripting engine.
- A wide, adaptable, and capable community.
- Responsible administrators and moderators.
- It only costs $20.00 USD (~$37.00 CAD, may vary due to value fluxuation)
- The game is almost unhackable, if not completely.
Cons:
- Brick physics are not permanent, although still there.
- Servers require forwarded ports and/or a capable UPnP connection.
- Bricks cannot float in the air (they can float in water, though), overlap, or be placed in unstable and/or already occupied space.
- Easily-spammable emotes and other things like macros.
- A lot of spammable, overpowered, and/or overall stupid weapons/tools/other add-ons.
As you can see, both games are good AND bad in different ways.