I would see no reason to go with the larger pipe turned back upwards. Once any bubbles reach a point where they are going up with the flow, they will quickly move on up and out to the tank. I find it very nice to have a flat bottom to set the reactor on rather than have fittings there. To avoid this I use a tee at the bottom end with a flat plug rather than a cap. Then I go with the adapter to get down to a barb fitting.
I find the restriction to flow is not so much from the turn, although that does add to it, but looking at the ID and OD sizes. With 5/8 iD tubing putting a 5/8 barb sounds like keeping the same size. But in fact a 5/8 OD barb has much smaller ID size. We go from a size that we can almost fit a thumb in down to a size that a little pinky won't fit?
If it was a highway, going around a curve slows it down but the fitting works like going from 4 lanes to two.