Magic Resist, or magic resist per level Glyphs?

In this case, you're probably better off with the flat stats, because the scaling ones won't catch up to the flat stats until level 9. Even for a solo laner, that's a large portion of the game to go with substandard MR.

The general rule is that if the flat runes are 3x less than the level 18 scalings (meaning they're the same worth at level 6), then it's recommended for solo laners to bring scaling runes. Bottom lane and jungle should always bring flat runes, because they take longer to hit 6.

Flat magic resist = laning against AP Scaling magic resist = not laning against AP.

If you're the jungler, since you're not directly facing an opponent and since jungle monsters deal physical damage, scaling MR will be more important (you can build MR for baron later).

Here's the problem, when playing squishy champions like ADCs, getting flat magic resist will help you survive better and trade. I know that Leona seems like she does no damage, but since you're full squishy, the flat MR will help you more than you think. FURTHERMORE, don't think 27 MR late game will help an ADC in any way. ADCs can get 1-3 shot, getting those scaling will be mostly useless. You can also say the same for squishy top laners and many many more champions.

In my opinion, you should get both and use both runes like me, but if you're only going to purchase one set, get the flat MR.

Source : countless days of research and going over challenger/diamond rune pages.