So, I picked up a 5500 FW (Fire Weather) and a D3 DROP FW as a package deal. I've learned a few things that might help someone.
Get one with LINK, LINK adds $60 to the price, but the data cable to update firm wear is also $60, so with just buying one unit you probable will not save money in the long run getting one without LINK.
The Kestrel is rated to -14 F, this is due to the LCD screen, which starts becoming sluggish around 0 and will be blank around -14 F. If you have LINK, and keep your phone warm, the Kestrel still collects data (so long as the battery works) and you can read that data on your phone.
The Kestrel uses a AA battery. If you keep the unit warm, it takes a bit to give accurate data, particularly if there is not a wind.
-Alkaline batteries do not like being cold, I would say just don't use them.
-Lithium batteries are rated to -40, however they do not like being cold soaked. An hour at sub zero temps will result in a small loss of power (100% charge to 95% charge), but an overnight cold soak results in a very large loss of power (100% charge to 5% charge). One could pull the battery and put it in a pocket, then reinstall it when needed. But, who wants to pull gloves off to install a battery at -30?
-NiMH rechargeables (Eneloop) are rated to -40 and do not mind being cold soaked. So, in subzero weather, NiMH is the best battery choice.
D3 DROP
-The FW version is the only D3 that does not give density altitude.
-If you need readings at about 10 degree or lower, the DROP is a bad option. The coin cell battery in the drop does not stand up to cold. I hung it outside in below zero weather with a full battery, before the temperature senser had time to equilibrate the unit stopped logging.