Is one considered better and if so why?
They each have some advantages, but in terms of quality, both of the ones ive had were a wash for which was better.
I personally prefer the A-bolt, but its not actually "better"
The Xbolts are noticably slimmer thru the middle, less angular, and are lighter. Effectively the same size X-bolt feels like a smaller gun.
The DM is quicker to access/swap as its external, and you can get aftermarket/extended mags.
They also changed the cocking system and did away with the little roller thats a ware point on the A-bolts.
The Abolts are also parts guns, much like a savage. You can swap pretty much all the parts (besides barrel) in mins. Within an action length family, bolt head, magazine, barrel are all thats required for a cartridge change (and some times dont need to swap mags)
The quadlock optics mounting system
@Hugger-4641 mentioned is also noticably different, and when it came out i assumed there would be issues keeping the holes straight (Remington has issue with 1/2 as many straight down the middle), but ive not heard of any.
He may think of some other differences also.
Oh one other note on A-bolts. They used 2 receiver threads, dont remember them off hand, but the stainless ones can have galling issues during installation making the barrels very hard and in some cases impossible for an average gunsmith to swap.
I did my buddies 99-2000 a few years ago and by then they had gone to the courser threads and we had no issues.