[Grouse Gunner]

Essentially the same gun, with different "engraving" and finish. I'd prefer the adjustable comb, as it really lets you "dial the gun in" for fit and POI/POA, and you can make the slight adjustments needed in the long run as you age and gain/lose weight, etc.

I agree.

In fact, I'm looking for a 28-30 inch 20 ga. Beretta Sporter myself. You can't go wrong with either one. If the conditions are equal, along with the price difference not being an issue as you stated, having the adjustable comb would steer me to the SP1. I like the wood better on the Onyx, as most anybody would; but I value the comb more, especially if it is the B-Fast version. They are both used versions with good resale value, so whichever you pick, you do not have much risk of loss if you change your mind or eventually want to move on to something else.

One thing I would do though is handle both side by side if that's possible. Being 20 gauge and essentially the same gun, there isn't much chance there will be any significant difference in handling characteristics. But some slight nuance in stock dimension or wood density etc. might be noticeable to you. Other than that, pick one and enjoy it! You can't go wrong.