The Anabat Express and the Wildlife Acoustics SM4BAT ZC are both zero crossing recorders for leaving out on site.
They both perform the same function and give the same results - ZC files of ultrasonic calls, i.e. bats.
The differences are in the design, construction and weight of the units.
The Express is a nice lightweight unit (476g with batteries) that's easily and discreetly deployed and it has a built-in GPS unit. It is programmed via a computer and can be deployed for up to 14 nights.
The SM4BAT ZC is heavier (1.2kg with batteries) and more obtrusive and the GPS is optional (uses the same GPS as the EM3). Programmed on the front panel and easy to check and reprogram in the field. Has an integral security case. Deployable for up to 50 nights.
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 Wildlife Acoustics SM4BAT ZC.