While battery is important, there are other considerations to take into account.
In fact, pretty much the main units available offer similar battery specs (depending on usage).
Things I considered on top of battery life were:
Customer service (for when things go wrong)
Cabling (one cable for phone, kindle, camera etc.)
Ease of use
Usability (for example, switching off for lunch and back on later)
Compatibility with phone (I like to change things on the fly)
Compatibility with apps that I use - Osmand etc.
Charging on the go (Dynohub)
Types of maps in unit and their uses
(A detailed colour map on such a small screen is nowhere near as clear or as usable as maps on my phone, or some units allow searching by address and some not. Some units store maps on memory cards that may need to be changed or can become corrupted).
Charging time. (I've seen one unit that requires 8 hours charging for 10-12 hours of use!)
The world of gps units can be very confusing at the start. Fortunately, you have lots of time to learn about it.
*Inspired by your car gps being "so helpful". A bike gps is not like a car gps! In a car you'll rarely notice if it's taking the long way home, the quality of the road or if it is deliberately taking you on (or away from) all the hills. You'll notice on a bike!
Good luck!