Whilst BBs are usually a tiny bit cheaper than pellets to buy, it's not a big difference so it's not something which would really be a mjor factor in your choice, but it is nevertheless a small bonus aside from all the other perhaps more important considerations.
When it comes to the realism of replica semi-automatic firearms however, BB-firing ones do offer some clear advantages over pellet firing ones. A big one is the magazine feed mechanism. The vast majority of pellet-firing semi-auto replicas use some kind of rotary magazine to hold the pellets since a pellet has to have a specific orientation, this mechanism technically makes them revolvers under the skin. Conversely, BB-firing semi-auto replicas most often use a spring-fed stick magazine of some kind, since this is a simple yet effective means to feed spherical ammunition which obviously does not need to have a specific orientation when in the clip or the breech.
This loading difference might not seem that important, however, if you are doing tactical shooting and using tactical reloads with a CO2 semi-auto replica, either for fun, or as an inexpensive and slightly safer means of practicing for when you use the real thing, then that difference in magazine operation is relevant since it is more akin to the real firearm. With replicas, the more realistic the functionality, the more enjoyable they tend to be since that replication is kind of the point, likewise the replication of slide blowback. More realistic replication gives a more visceral experience akin to the real firearm, which is kind of the point with a functioning replica.
Aside from the fact that this is more true to how a semi-automatic firearm operates, making these replicas a bit more like the thing they are replicating, it also means the round capacity of BB spring fed stick magazines is much closer to the capacity of the real firearm. For example, the Umarex BB-firing Glock 17 has a magazine capacity of 18, and this perfectly replicates the firearm Glock 17's magazine capacity of 17 (which is where it gets its name from), with an additional one in the breech ready to go (not always, but a common procedure), for a total of eighteen. You can then load spare mags with 17 rather than eighteen BBs, and operation will be exactly as per the real thing in terms of shot capacity and reloading.
By comparison, the pellet-firing versions of the Umarex Glock 17, and the version which can fire both pellets and BBs too, use small rotary eight-shot magazines. Of these, whilst the main ejectable magazine is comparable in size to the real Glock's magazine because it is designed to conveniently store three additional spare rotary mags clipped into it whilst also holding a fourth one which is the actual operating mag, this means it is only an eight-shot pistol before you have to eject the main magazine then swap over the rotary mags within. One of these rotary mags which comes with the dual ammo capable version is designed to hold BBs in case you were curious about how that dul capability actually functions, but with either BBs or pellets, it's only ever an eight shot pistol between reloads.
But all this stuff is about replication rather than accuracy. So...
Both the dual ammo capable and pellet only capable versions of, for example, the Umarex Glock 17 have rifled barrels, and this means they will be a bit more accurate with pellets. However, it's probably fair to say that at the ranges one would typically use a semi-automatic pistol (either real or one of those Umarex replicas), the difference in accuracy is likely to be fairly minimal, because if you want perfect accuracy, these are not the kind of pistols you'd choose, so really, BBs are more than accurate enough for the kind of shooting you'll be doing with these things.
But - and this one is really important - whilst you can sometimes get ricochets from lead pellets, you do tend to get them a lot more from BBs. I'm sure you know that's because lead pellets compress on impact and that spills a lot of their energy, whereas steel BBs don't compress very much at all upon impact and that makes them more prone to bouncing off whatever they hit. It also means that BBs tend to penetrate hard targets more than pellets do; a good example being glass bottles: Shoot a glass bottle with a lead pellet and you'll more than likely smash the bottle, but shoot it with a steel BB and it will probably put a couple of holes through the thing.
This means two things in terms of safety, one, always wear shooting glasses when shooting BBs (you should do this when shooting anything to be honest), and always use targets which are designed to trap and collect spent rounds and use a big backstop behind these targets which does that too, because if literally any pellet or BB ricochets and leaves the boundaries of your property, aside from that being at best annoying and at worst dangerous, it is also illegal. Not a big deal if you are at a range, but certainly something to consider if you will be plinking cans in your back yard.
So personally, if you want a replica semi-automatic CO2 air pistol - and you should if you like shooting and are interested in weapons, because they are great fun - I'd say BB was the way to go simply because they tend to have a magazine shot capacity closer to the real thing and their magazine mechanism is more akin to the real thing too.
