I think it depends on how you fish. I have a 21' Kenner and went with a 112 lb thrust. But I use my motor almost all day with strong winds much of the time. If you plan to use the motor to spot lock in heavy winds or current, or you are going to run it 10 to 12 hours a day, I think you'll be happier with a 36 volt TM. If not, then 80 is probably enough. I doubt you'd be happy with less than that for anything more than occasional use.

If you can elaborate more about how you plan to use it, we can help you better.

 

If buying a trolling motor (new or used) there is no benefit or savings to go with a 70. If you are going to go thru the trouble and expense and weight of adding batteries, wiring, trolling motor mounts, quick disconnects, the teeny weeny upcharge (if there is any) is not much at all in the overall expense of adding a trolling motor.

Max thrust for whatever voltage range is the only consideration. Anything else would be a do-over at some point in time. Do it right once.

 

I'm with sgrem... The upsize adder on top of the cost of a new 20' boat is less than a percent of what you've paid for the rig.

I'd go further and suggest going to 36V unless the space for batteries is tight. You can always run 112# at a lower speed and get a few more hours (and miles) out of the 3 batteries. You can't get more than 80# out of an 80# troller.

Edit: To me, the big question is whether you want to spring for the auto-deploy.

 

I hardly ever go out any more in bad weather including wind so I didn't get the biggest on my boat 'cause I wouldn't ever need it, but there was a time in my past when I couldn't decide when to go and had to take what was dealt ... I'd a gone bigger then ... if you're in a 21ft boat the 80lb will probably work for you 90% of the time and it'd be up to you to decide if the extra $ for going big is worth it ...
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I would put a 36v-112 myself. But if you only have room for a 24v, then I would only do the 80. I have never said I wish I had a smaller trolling motor. I have a 80 On my 14ft poling skiff and a 112 on my x22. There are days I wish they made a 48v. When you build out the boat if you do a 24v you will add a 3 bank charger. If you decide to upgrade to a 36v you will have to add a charger or will have to upgrade to a 4 bank charger later and will cost you that much more later on down the road.

 

I've got a 16' flats style aluminum rig and I am so glad I went with the highest thrust 24V unit I could get. 80lbs Minn Kota Terrova w/iPilot. I can hold in current below the dam at Lake Livingston all day long.

 

I run a 80# with thousands of hours on a 21'. I haven't ever had a problem unless I've had a bad battery. A lot of guys are going to 24v systems from the 36v systems because they've gotten better over the years. I don't see a need to do a 36v on anything over 22' depending on weight and style of hull.

 

Sgrem nailed it. The extra cost of 10lbs of thrust is negligible. if weight and space considerations dictate that you cant go with a 36 volt, then a 24 volt system with great wiring and group 27 or group 29 batteries will perform well and give you a full day of fishing in most cases. I still see more issues with self-deploying troll motors than manual deploy and stow.