I'm not a fan of whole life because all it does is add cost to term insurance in return for putting some of your premiums into crappy investments. You can certainly get a level premium term policy.
I don't think there's any strong argument either way for 20 or 30 years. I support the concept of buying insurance for when you need it, and once you're retired and have sufficient assets, stop. Just went through this with my agent. The term policy I have is set to go WAY up next year after the level 20 year period expires. I said I'm just going to drop the policy, and he gave me that fake concerned look and said something like "You know, if you die, I'm going to be the second person your wife calls - and what will I tell her? You dropped the policy and there's no money?" I said my wife will be fine and not to worry, she won't be calling you.
And regardless of how you compute the amount you want to be insured for, please caution your wife to NOT pay off the house if you die. It makes no sense to take a large pile of money and sink it into your mortgage where it's now locked up and unavailable, even at the cost of some interest on the mortgage. Remember, by that time the amount of interest per payment won't be all that much.
If she needs the money, then she either has to move (cash out) or borrow it back on a HELOC and pay interest. Illogical. Keep the money, make mortgage payments like usual for at least a year. She might want to move, she might not, but most times the rush to pay off the mortgage is a poor decision.