In the long run it is probabley a better choice without the urchin.
Here are some good snail choices:
great for detritus and the sand bed.
especially good at film algaes..
cleans both the glass and the sand bed..
cleans up the uneaten food bits as well as detritus... looks cool too.
another excellent algae eater..
All have a different function (some similar) and will aid in keeping the tank clean.