IMO the RB2 is more desirable and a better functioning gun than the later tap loading Airsporters.The MK1 and MK2 are the only tap loading Airsporters worth having as the build quality,fit and finish were very good on these,and both models had a properly lapped in tapered tap which when new were airtight.The taps on the MK1 and 2 automatically opened when the underlever was cocked.
The MK3 Airsporter had a tapered lapped in tap,too,but the automatically opening feature of the tap was dispensed with on this and all subsequent models and had a less elegantly styled stock and cheaper open sights.
Later marks of Airsporter were significantly cheapened in production to the point of becoming disgraceful considering it was the flagship BSA model!The taps on the MK1V and all subsequent marks until they ceased production were parallel and rarely achieved the fine alignment and airtight seal of the earlier tapered taps.
Collectors,of course do find the Stutzen tap loaders desirable,but I personally would not bother with any of them!