Your choice. The Optima should have wiring grommets on both sides, adjacent to the side with the main controls. You can also come up thru the bottom.
I went the lazy way and ran my conduit above ground- I didn't feel like trenching. Underneath makes for a neater less cluttered install. If you leave enough clearance around the spa most service should be done without needing to move the spa, I'd think.
Base your decision on what you like better, or want to pay for. If you're hiring this out, get advice of electricians for cost of one way vs the other. I suspect that trenching will add to your costs.
You'll need to plan this out before installing your concrete base.