0.5% assault per skill lvl, so assault (large) hits 16% at lvl 20 while assault (medium) hits 13% at lvl 20.
It is a low difference, but keep in mind:
1. SSRs generally will have higher atk, so you prefer them eventually.
2. Certain SSRs later get the option to receive a final limit break later. Part of the prerequisites include being lvl 125 and skill lvl 20. This FLB raises the lvl cap to 150 and skill lvl cap to 30. Also, either a pre-existing skill is strengthened or a second skill is added. Other resources are also needed.

So yes, a lot of resources are needed to fully realize an SSR's power. But they're also meant to have the highest ceiling, so it's a fair tradeoff. SRs are quicker to reach their ceiling, but that ceiling is also lower.

Essentially, you invest in an SR with the understanding that it eventually gets replaced by an SSR. How long until that happens? That's variable, but can be lengthy of the order of many months. So you're striking a balance between opportunity cost and your immediate needs. And when it comes to opportunity cost, constantly readjust. Things like 'oh hey, my grids are full of SR or above, any new SRs from this point on is fodder' changes your situation. Things like 'union events from now on spit out tons of R cups' changes things. Or the recent gem gacha update on DMM that removed N trash changes things (by the gods I want this update already).