When in doubt go smaller. An 80K/80% is 64K out, which is literally twice what I need on a similar-sized house with even less attic insulation at -5F. Knowing the ratings of your existing furnace it's possible to use it's fuel use correlated against heating-degree-day data to come up with a fairly precise estimate (more precise than any heat-loss calc based on construction, since it's a measurement not an estimate.) Most heat loss calcs use "typical" air infiltraion and sloppy R-value/U-value estimates and pad it to protect against undersizing discomfort, which ends up spitting out a number 25-35% over reality, then the contractor goes the next size up from that "for good measure". Odds are your real design-day heat load is more like 40K than 64K, which would make the 100K/80% literally 2x oversized, and the 80K/80% unit 1.6x oversized, which is close to the oversizing assumption in an AFUE test, so it'll actually DELIVER 80% efficiency, whereas the 100K/80% unit will be delivering between 75-78% as-used efficiency. The 80K unit WILL be more efficient, and more comfortable, but not as comfortable (and far less effiient) as a 2-stage condensing furnace.
Condensing furnaces just aren't all that expensive, and even without subsidy the payback is good. A 95% AFUE furnace, perfectly sized to the heat load will deliver slightly better than 95%. At 1.7x oversizing it'll use 18% less fuel than an 80% unit per year. If you're currently burning through 1500 therms/year with the 65% beast you'd burn 1220 therms/year with the 80K/80% furnace, but you'd burn ~1000 therms with a condensing furnace. At a buck a therm (it may be more, it may be less where you are) the annual difference in operating cost would be $220, or a 5 year cost difference (assuming no fuel price increases) of $1100. If you look at the difference in up front cost, and the after-tax return on that difference it's usually quite favorable compared to other places you might have invested the money. Don't forget to factor in any state/federal/utilility subsidy for going with the higher efficiency unit too.
Bear in mind that if you lator add insulation to the attic or make other envelope upgrades your heat load goes down, which makes your oversizing factor go up, reducing net efficiency. But with a properly sized 2-stage condensing unit the efficiency extends much better than with a single stage unit, and you'll be taking fuller advantage of the insulation upgrades. (In fact some 2-stage condensers run slighly MORE efficiently when oversized 2x, since the higher stage burner runs at higher temp, reducing the condensing factor, but oversizing does result in reduced comfort.)
But the short answer: Go with the 80KBTU unit, if those are your only choices.
