I’m no expert by any means but the advantages of painting the underside as opposed to under sealing is that you will see if any rust starts to appear quite quickly with routine inspections. With under seal you could have all sorts of nasty things happening underneath the sealant if water gets through and you wouldn’t know it as ive found with mine while I have been striping it.

If you want to paint it seems that 2pac offers the best finish and goes on thin so it looks great, its still very durable (not as durable as powder coating though) but its expensive and requires specialist equipment and it is toxic so most paint supplier wont sell it to Joe public. Im not sure if standard automotive paint would hold up underneath the car with stone chips etc.. Maybe with enough layers of lacquer it may im not sure..

Powder coating needs an oven to bake it on but its the toughest paint finish available so ideal for suspension arms etc. Ive had this done to mine