Well, I,m sorry, I read your post too carelessly. It's pretty obvious you don't have one.....it would be easy to spot and would currently not be tied into your other returns (or shouldn't be).
So......
Cost - A pressure side cleaner will cost you the new plumbing, a new booster pump and the cleaner itself - perhaps more than $800 or so. A robot doesn't require anything else.
Secondly, you pay for cleaning power in most all types of autovacs. The more it costs, the better and faster it does a job.
For a pool your size (and assuming you get a normal amount of usage and dirt) I would suggest the cleaning power of a robot....you're just gonna' have to pay quite a bit for it.
I am a bit confused by your pool cleaners response because you currently do not have the piping required for a pressure side cleaner. If you go that route, however, pressure will not be an issue.