Which one is for the detailed finishing work? Never heard of micro ballons before.
Bondo is going to be easier to sand, but the micro balloons will not absorb water. The micro ballons act simply as a filler. Mixing the balloons with the resin thinkens it up, turning it into a paste. Mix 4-8 oz of resin and add balloons until you get the consistancy you like (4 oz of resin + balloons = 8-12 oz of product). Too much balloons and it can crumble, too much resin and it will be runny. It's not a very exact science, it's very easy to work it. Just don't like it dry for 2 weeks and go back to it. It gets extremely hard and it's a lot of work to sand after a long drying time (sand after about 24 hours give or take).
Anyway, it worked very well for me. I ended up with a lot of pitting after I sanded it smooth, added a single coat of plain resin sanded with a lighter grit paper and it felt and looked like glass w/ a very glossy finish before any paint.
The baloons are fairly cheap, and you already have resin. Guarenteed to stick because it's epoxy, wont absorb water because it's resin and glass, can be made very hard by using extra resin (if working on the bottom hull like I did) or very soft if just putting over foam to lay glass after. I like the stuff...
