Personally, I think a fountain pen writes better than both a roller or gel [] But since you have allready disqualified my answer.....
It is all personal taiste, buy both of them and see what YOU like the best. Also there is a BIG difference in BRANDS and TIPS and the PAPER you write on. I ussually will use a roller when the FP in not at hand, which is rare. But I have tried rollers that are awefull. The best roller in my opinion is a refill made by LAMY, by far the best. I have been trying to make a pen for them, they will not fit our kits.
Being left-handed I prefer a ball-point but have several gel pens that I manage not to smear. Also I write about the size of this type so inks that spread or have a wide footprint are a problem for me.
I recently purchased several different brands of rollerball and gel refills and there is a big difference between brands. SO thats a question that only you can answer.
Keep this in mind -------- Strictly my OPINION---------not scientific!
I have NEVER been happy with any rollerball I've owned. They have all skipped from the get go. [V]
I replaced the refill of an Americana Rollerball I made as my first pen with a GEL Refill. Couldn't be happer.[] BTW, being LEFT HANDED, Fountain pens are out of the question----------very messy.[xx(]
Did you know that "Gel" ink can not be washed from a check? Regular "roller ball" ink can be washed from a check. So If one writes their checks with a Gel pen then the check cannot be changed if it is stolen. So Gel is better in terms of archival quality.
I find that the gel refills are a pretty expensive way to write. The ones I've tried go dry in about the same timeframe as a small fountain pen cartridge ... but cost a whole lot more! I've tried rollerballs and have similar concerns about them. I really like the way both of them write, but I'm not willing to pay the premium over a ballpoint. I have also noticed, however, that both rollerballs and gel refills tend to skip on shiny papers (like the thermal paper credit card slips) or paper that's picked up oils from my hand. It's ballpoints and fountain pens for me... []