It is hard to tell which one is the best, since they don't say much
about TEG limits ( hot cold side ) and how they expect the TEG to last
with wear and tear. ( I have one pot from each company as well as a
bilote-stove and some more stuff ;-). So far I can just say I'm most
disappointed with my biolite stove ( mostly because it's very heavy and
impractical ...but I think heating a rod is a good tactic to avoid
overheating ...but still I have lost interest in it). The pots are very
likely to sooner or later be overheated, I try to treat them nicely (
planning for a temp guard ).
I think TEG with high max-range ( hot
cold side ) and a pot with large surface area and "large" watervolume
AND that is "well applied" to the cooling surface is a good start..

more solutions
http://www.tegmart.com/thermoelectric-generator-products/

maybe have a thermocouple that gets the temp of the hot side, and if it gets too hot, a mechanical valve diverts some of the water cooling from the cool side to cool off the hot side.  that would prevent overheating.  and yeah, maybe use a metal rod to conduct heat out of the fire to the hot side rather than sticking the whole thing right in the fire

If you have a "connection box" outside, you can switch parallel / serial  ( for the preferred voltage ). The key to success is reliable cooling...My guess is water is the best option for a portable system. The "only" relay difficult part is how to make this rugged and reliable, especially if you plan to use fire directly from a wood-stove. ( the biolite solution is heavy and requires moving parts..and small top-fed stoves tend to be very impractical )

Module - 19 Watt 4 Volt 56mm
http://www.tegmart.com/thermoelectric-modules/tegpro-19-watt-high-temperature-thermoelectric-module/

and don't forget.... ;-)
http://www.electronics-tutorials.ws/dccircuits/dcp_9.html

so far these two is the best I know of 

http://tes-ne.com/English/product/

http://powerpractical.com/collections/generate

those are some impressive generators in the 2nd to last link, if they work as advertised... but at what cost?

  especially that hot tub one