the difference is return on investment. but, there is a minimum roster level requirement to be able to make the most of the pvp rewards. if you have at least a decent 4* pair, you could end up with 78 cp, 3 4* covers, and at least 9 3* covers every week (t50 and 1300+), along with 9 tokens. all for much, MUCH, less time than a week of competitive pve will provide. however, this usually takes some shields, so for many, pvp isn’t very hp positive, and is an hp drain for some. per match, there is more iso in pvp matches as well, although to play high levels, you’ll be spending some iso on skips and boosts. so pvp isn’t an iso/hp mine if you’re playing at high levels. you make some iso, but it all depends on how much skipping you have to do to find points/matches.
pve week (t50) will result in 9 tokens, either 39 or 64 cp (depending on 1 or 2 events), 425 hp (350 for sub placements plus event progression), and lots of iso (as long as you minimize boosts). the total iso is considerably more in pve because of the volume of matches you must complete each day (~50-70 matches per day), even if per match, it is less.
the most effective way to not kill yourself is pick the one you can do well. if your roster is a decent level, focus on pvp and just do what you can in pve. if you can’t at least get to 1K in pvp, focus on pve and just do what you can in pvp. top players do both consistently, which if you can handle it, will blaze you through progressing. other than iso - that lacks more than any single aspect once you’re working on 4*s (and can hit 1300).
note: xidragon brings up a good point. pvp also has season progression and placement that adds rewards along the way - covers, a 10-pack at 4K points, an LT if you make it to 10K. and pvp also has shield simulator, which adds even more stuff.
I probably overlooked some aspects, but that gives you a general feel.