Lotsa variables here, so it will depend on your goals, and your definition of "simple performance upgrade". You need to take into account the whole engine and not just the block in order to make a choice.
Are you looking to conceivably get a block that you may not have to bore, to just do a scuff and re-ring job? Then Magnum or roller LA.
Four barrel and manifold? LA- huge variety of intakes, tons of used ones out there. Magnum- only two choices- Air Gap and clones (okay, there's the Indy ModMan, too. But $$ and not that great).
Heads? LA- lots of good factory heads and tons of aftermarket. Magnum- prone to cracking. Plan on getting replacements, although there's a few starting to show up in the boneyards that already have replacement heads.
Cams? (Ignoring current availability issues) LA- run whatever you want. Hyd., solid, roller... '88ish- '91/'92 came from the factory with rollers. Magnum- most off the shelf magnum cams tend to be RV grinds, since they were originally truck motors. But the selection is getting better in recent years. "Short nose" cams, no provision for driving a mechanical fuel pump.
If you want to know the truth, there is fundamentally no difference between the late LAs and the early Magnum blocks. Late LAs had factory roller cam provisions identical to the Magnums. The early Magnums were still drilled for rocker oiling, it just wasn't used (after a couple years they stopped drilling that passage in the block) so you can still use LA heads on the early Magnums. The Magnums had the truck mounts cast in the sides in addition to the LA mounting ears. The LA 273/318/340 had identical oil pan end seal diameters, LA 360 and magnums had "big and littles". Otherwise VERY little difference in the blocks themselves. Your chance of finding a good block are better with the Magnum simply because they're newer, and the FI does a better job preserving the bores. The LAs are older and may have multiple rebuilds under their belt already...
Your choice will depend on your intentions.