I would look at the technical specifications to see if you can see what chipset is used, i.e., SiRFStar III, or Qualcomm MSM7200 gpsOne. If you know the chipset you can look around and see if there are reviews of units using that chipset that either talk about problems or slow speed locking onto satellites, or ideally the reverse. I have the SiRFStar III chipset in my smartphone, and it does a good job of locking on fairly quickly under tree cover. Maybe all models sold today will do so, dunno ...

My recollection is that some GPS models don't (or at least once didn't) work with lithium batteries; this is something I'd want, particularly if you anticipate using it in cold weather.

If buying based on maps that come with it, be careful about the scale of maps included. A friend got a gps (Garmin Colorado I think?) partly based on it having all of the US included, but these were 1:100,000 scale maps. I infrequently like having the ability to see this higher-level (less detailed) view, but I almost always look at maps (printed and on my phone/gps) at a 1:25,000 scale, and to get that level of resolution my friend had to purchase the maps (they're not cheap, either).