Sell stock to buy similar Roth IRA?

I'm trying to wrap my head around this, but in my mind I'd be looking at similar results.

Right now I have a decent amount in a stock brokerage account invested, let's say in VOO.

If I sell, I take 15% loss on the gains. Then invest into my roth IRA to max it which is post tax.

If I leave it in, I get taxed at 15% at retirement.

It's the same... right? Or am is there something I'm missing that I should take advantage of?