What are the pros and cons of undergravel filter? I have one, in addition to my aqua clear filter. I am trying to make up my mind whether or not I want to keep the undergravel filter there or take it out completely and rely just on vacuuming.

Personally, I would take it out completely. With an undergravel filter it is still necessary to vacuum sweep the gravel, however some of the fish waste and debris will fall through the slots and become trapped beneath the filter grate. Over time, it (the debris) could become toxic. The only way to effectively get rid of the trapped debris is to remove the undergravel filter plate so you can vacuum up the material.
The one pro I can think of is the surface area of the gravel makes a good biofilter, but I would suspect the difference between a UGF and straight gravel is small. It would also depend on what you use to run the UGF (powerhead or just airstones)

If you are able to extend the intake tube of the aquaclear to an inch or two from the bottom, it will suck up a good part of the debris that would otherwise be stuck in the gravel.

I have tried it all and the best solution I have is barebottom. Especially with goldies.

UGFs are yucky!!