According to AT Bench, 830 owns the Heavy Workload tests; 840 owns the Light Workload. Synthetics aren't worth much imho so whatever on them.
The Heavy tests are write-heavy and the 840 is weak especially on sequential writes... so if you're transcoding, decompressing, writing crap tons of video, or just installing giant programs for fun, the 830 is the way to go.
The Light tests are read-heavy and that's where the 840 wins. So if you're working in Office, concerned with loading apps, games, game levels/data, etc then the 840 might be a better option.
Looks like the 840 has some minor power advantages too. Might be worth considering if you have a lappie... if it's a desktop the difference will be like 4 cents a year.
I'm debating between the two myself. I have a lappie and I do a lot of productivity work and some light gaming on the side, so the 840 actually is a better match for my workload - the 830's better write speed will save me a few minutes when I wipe my drive and reinstall stuff, but that's about it. And a bit more battery life never hurt anyone.
The tradeoff is that the 830 128GB right now is 90 bucks and the 840 is 110... and if I go with the 840 I'm losing 8GB of space. Not a huge deal, but it does push the price per gig equation a bit farther in the 830's favor. It's a much smaller deal at the 250-256GB size since you're losing a much smaller percentage of your space (and only 6GB instead of 8).
I'm not horribly scared of the 840 having reliability issues - the controller is an incremental improvement on a proven design, and Samsung's TLC NAND is similarly proven. The 830 was solid from Day 1 and I'd expect the same here. Still, having the extra proving time couldn't hurt.
I'll pry order the 830 later today. My guess is that the light workload advantage won't even be noticeable to me since it's, well, a light workload. I'm just sick of waiting for the bog slow 5400rpm drive that came with my laptop... pretty sure the Pony Express would be an upgrade from this thing.