Towing with a Recumbent Trike, Tadpole or Delta?
Before you buy one, ride a couple of different trikes. It's worth spending $500 doing that before spending $5000 on buying one. You might be able to find a local recumbent group, or a retailer with demo units. The IHPVA is a good start for the former, or the usual social media.


  half as much load per tire...


Trike weight distribution and handling is something that varies a lot. Most of the delta trikes are rear-heavy, tadpoles range from equal to rear-heavy, with extra rear weight more common when they're loaded up. Rear biased does tend to encourage understeer, but for the most part those trikes are the less sporty ones (and for heavily loaded tadpoles the rear wheel will probably fail catastrophically before losing traction... I think, I've never actually seen it happen).


  I primarily ride on 'rails to trails'...  my trailer is around 100 pounds (45 KG) ...


It sounds as though you already have the trailer, so that just becomes part of your filter criteria. You have to decide whether the total cost of "trike + trailer adapter + shipping" is worth it to you. Obviously if they don't have the adapter or can't ship to you there's a bunch more cost and thinking involved. But Hase and Greenspeed delta trikes both come with trailer hitches, so it's not unusual.


  For towing is a Tadpole or Delta the best option?


For lightweight loads like the ones you're talking about it doesn't matter. That's well within the design limits of all the commercial trikes I'm aware of, and it's quite reasonable if you're building your own. You're better to focus on a trike that works for you in other respects, I think.