You can probably get by with just a standard Kestrel without the AB, since you have the Garmin.
That being said, I'd recommend you get the 5700 Elite with AB, just in case your watch takes a crap, if you can afford it.
Two is one, one is none.
It's why I have a Garmin 701 and a Kestrel 5700 Elite. It's a paltry price to pay when you have gear go down, and you've already dropped $2K+ on match fees, travel and lodging. Same for a hunting trip.
JMTCW...