Hi Dave,
Buy the Beretta - the trigger alone is in a different league to the CP99.
As far as power difference is concerned, I seem to recall the '99 putting out around 360 FPS, whereas the 92FS claims around 460 FPS in the manual, so yes, it's a little more powerful.
People on here have fitted over-barrel silencers to their Berettas, although I don't honestly know how much quieter they'd make it. Bear in mind that those type of silencers will leave nasty grub-screw marks on that nice satin Nickel finish - if you're happy with that, then go ahead.
Expect to pay around the £100 mark for a boxed, good condition used example - you should get some CO2 bulbs and a tin of pellets thrown in at that price, too.
With regard to the laser, it depends which laser it is. The Umarex trigger lasers sell from between £45 - £60 new, and a Beamshot will set you back around 45 quid. I'd say £25 or £30 was a fair price for a secondhand laser.
If you have the means to buy it I'd jump at the chance - you will not be disappointed.
HTH,
Mike.