These units are all full spectrum ultrasonic recorders for leaving out on site.
The Elekon Batlogger M can also be used hand-held for active surveying, though they don't have a sonogram display.
The Elekon Batloggers A+ & C, Peersonic Weatherproof, Pettersson D500X and Wildlife Acoustics SM3BAT & SM4BAT FS are passive recorders. There is also the new Anabat Swift..
If you want a unit that can be used both actively and passively, the Batlogger M is an excellent piece of kit.
For passive use it's a matter of personal preference (and budget), as well as how you'll use them (see below).
Choose whichever one has the features that suit you best and if you need some help choosing, call me on 0333 9000 927 (+44 333 9000 927 from outside the UK) or email me.
You'll get a free bat calls book worth £30: either British Bat Calls by Jon Russ, Social Calls of the Bats of Britain and Ireland by Neil Middleton et al or Bats of Britain & Europe by Christian Dietz & Andreas Kiefer; with the Anabat Swift, Elekon Batloggers A+, C & M, Pettersson D500X & D1000X, Wildlife Acoustics SM3BAT & SM4BAT FS.