If you are set on one of those 2 guns, buy the one that will shoot the POI you want or need. If you don't know exactly what POI you want or need, I would by the adjustable gun. The adjustability will save you in the long run and will also likely have a better resale value in the future.
Sam, I'm not sure which guns you are asking for an opinion on.
The MX-2000/8 is an MX-2000 receiver with a top single, fixed rib barrel that was originally offered on the MX-8 Trap.
An MX-2000 is an unsingle only gun, with an adjustable rib. Rib height is higher than the MX-8 Trap.
The MX-2000RS and the MX-10RS come with an unsingle, adjustable rib barrel. Rib height is higher than the MX-2000.
Next is the MX-2000/10 and MX-10 come with a top single, adjustable rib barrel. Rib height is higher than the MX-2000/10RS.
I prefer adjustable ribbed barrels, but they are more expensive than a fixed rib barrel.
The barrels are all the same. Only the type of rib and its height are different. The adjustable ribs let you tune POI by relatively small amounts on their own. For more adjustment you need an adjustable comb. Once you have an adjustable comb, you can tweak POI by a little adjusting only the rib, or a little more by adjusting only the comb, or a lot adjusting both at the same time. That is how I use mine.
However, for many, the best reason to own an adjustable rib barrel is so they can maintain bead alignment when they adjust POI. If you don't care, and don't mind possibly seeing a little rib between the beads, then a fixed rib barrel and an adjustable comb will give you everything you need, especially if you use the Perazzi spacer set with the pair of 1mm rings.
ZZT is almost 100% right.. The MX2000/8 can be had as a combo.. It's nothing more than a MX2000 with the MX-8 fixed rib barrels..Literally a pretty MX-8.. The MX2000/3 is an adjustable rib O/U with a lower rib than the MX2000 or MX2000RS.. If you perfer a flatish rib.. either do the same job.. but the MX2000/3 IS adjustable and offers some change in POI.. Both are excellent guns.. The MX2000/3 would be excellent for both ATA and Bunker trap if you shoot anything other than 50/50.. I own a MX2000/3 and love it.. also quite a few MX-8 step rib guns.. For me.. both serve exactly the same function..
Having said all this.. I'm working with Dennis on a low,adjustable.. short rib for a MX O/U.. Still in search for MY ultimate trapgun.. Hope this helps..
one last thought.. The MX2000/3 can do everything the MX2000/8 can do.. and then some.. That can't be said for the MX2000/8 On the flip side..Everything is still trying to catch the number of old MX-8's that dominated the winners circle..all over the world..in every type of shooting..
Yes, the most obvious advantage is the adjustable rib. There is another, less obvious advantage, a higher rib.
If I had this decision, I would buy the MX2000/3 every time. Not because it has an adjustable rib, but because it's higher. Where I live the heat waves off the barrel bug me with my low rib guns. My high rib guns, not so much. So, I like the MX2000/3.
Of course you can get just about anything you want from Perazzi. They will match any barrel to any receiver. You just have to ask.