Benefits:

Bit more thrust, 2 to 3 blades is not 50% more thrust, but its like 15-20% range.

Sounds more awesome. It is a benefit, I swear.

IMO biggest one, while 2 to 3 props don't give you 50% more thrust, you still have 50% more prop gripping the air, as people say. Movements feel faster/more accurate. When you cut throttle more area feels like a bigger air brake. I dunno hard to put in text, but the feel is biggest benefit.

Downsides:

More blades make more thrust, but it is less efficient, so less grams of thrust per watt of energy.

In general speed is combination result of pitch and rpm. More blades make more thrust, but they spin slower.... pitch is same but rpm is going down, so top speed is generally lower.

Same benefits/downsides exist for going from triblades to quadblades... and quadblades to hexblades.... but it's always diminishing returns. Most people end up liking triblades.

Cool thing about quads, you can go buy 50 different types of props and they'll all make your quad fly differently. Its not even 2 blade vs 3 blade, there are like 20 types of 5" triblades you can buy.

I say just buy 2 pairs of a few types of props and try them, experimenting is big part of hobby. :)