One thing that hasn't been mentioned yet (and if I'm stepping over a line here mods feel free to zap this post but I'm hoping I can keep it decent
) is the fitting of Anti-Tamper.
By and large, AT means these days the fitting of non-removable bolts to stop you stripping the rifle's action or doing any work on it. The idea is to prevent you from turning the rifle up above the legal limit.
For the individual owner though, it does have it's downside.
On some models, for example, the Air Arms S410\S400 (and the S510 too) it will mean you'll not be able to get into the action pretty much at all.
Why would you want to? Several reasons:
On the old S4x0 series there was a fine power adjuster (no I'm not going to say where it is or how to use it on a public forum) that allowed a responsible shooter who had access to a chronograph to adjust the rifle's power to ensure his rifle never went over the legal limit whatever pellet he used and to allow for variations in power due to seasonal ambient temperature changes.
In addition, if the rail on which the hammer which knocks open the cylinder valve got mucky (something that will eventually happen unless you're an indoor shooter!) it often led to inconsistency in power and that could affect accuracy.
Also servicing the S4x0 series is pretty straightforward if you've got a good set of instructions and some reasonable quality allen keys. The seal kit was available pretty cheaply and a chap with some common sense could do it himself and save himself the cost of getting the rifle to a gunsmith and the cost of the service itself.
AT, for the competent fellow, pretty much knocks all that on the head.
The story is similar to some degree or another on most PCPs made in the last couple of years.
If holding an allen key for any task other than putting a scope on a rifle makes you nervous then none of the above matters. A new PCP will be fine for you.
If on the other hand you want to be able to do some of the things mentioned above you'd be better off buying second hand.
As has been said the S400 is an excellent HFT rifle and can be picked up for quite a reasonable price secondhand.