Straight for sure. It is not IDEAL for long periods of continuous glassing, but no spotter really is. So where the straight really shines is swapping back and forth with your binos. Get an adaptor for you binos, and then mount the adaptor on your spotter so that it aligns perfectly with whatever was centered in your binos when you swap them out. It will save you a TON of time in the field getting quicker acquisitions of what you saw with your binos when you swap them for a better look.
In reality, and tons of field use, I would say I use a spotter for actually glassing, maybe 5% of the time. You do see some extra animals in that 2-4 mile range that would have been hard to glass with binos. But I have glassed a buck at 3.5 miles with a pair of 15's with his summer coat on and plenty of bulls beyond 4 miles.
Spotters are very glorified on TV and Youtube, but overrated for actual glassing in most circumstances IMO. But they are definitely irreplaceable for getting a better look at what you see with your binos.