The above discussion may give aviation buffs real woodies

, but as even one of them admitted "Either is 1,000 times safer than being on any CR road." So why are they even discussing it (other than the aviation woody part)?

Given the relative safety of flying even on a single prop plane compared to driving on CR roads (something we have all done with relatively little thought to its safety risks), that issue would be fairly low on my list of criteria for choosing which airline to take. However, there are a few other more practical and immediate differences that I'd consider first.
First, if you plan to catch a flight out immediately after you arrive in CR, Sansa is probably more convenient as it flies from a terminal right next door to SJO (assuming your arrival and departure times match up sufficiently. Even more importantly, if you want to head immediately out to PJ, Sansa may be your ONLY choice. Relatively few flights arrive early enough to even catch Sansa's last departure (at 2:15PM). In contrast, you'd probably have to arrive in CR by 10-10:30AM in order to clear I&C and make it over to the Pavas airport in order to cash Nature's last departure (at 11:30 AM). OTOH, if you plan to spend at least your first day or two in SJ before flying on to PJ (which I'm guessing would be most mongers' plans), then Nature Air's airport in Pavas would be slightly more convenient (and less expensive) than having to take a taxi all the way back out to SJO.
Secondly, there may be significant cost differences. REGULAR fares are pretty similar between those 2 airlines ($114 o/w including tax to PJ vs. $113 on Nature Air). HOWEVER, Nature Air also offers something called their "Loco fare" and it IS "crazy" cheap IF you can get it ($81 o/w). The catch with that is that the seats at those Loco fares are limited and they tend to sell out, so you usually need to book it well in advance. Also, the Loco fare comes with more restrictions as to cancellation or rescheduling(Nature Air also has another fare with fewer restrictions than their regular fare called "Elite" which would cost you $129 each way).
Thirdly, depending on your circumstances, their schedules MIGHT be a factor. Again, their REGULAR morning direct flights are pretty similar. Sansa has a 6:30 and 8AM departure while Nature departs for PJ @ 6AM and 8:30. Since you'd probably have to get up a bit earlier to get out to SJO that 1st flight is effectively even closer than it sounds. OTOH, if getting up at the break of dawn is not for you then that Nature Air 8:30 flight sounds a little nicer. Nature also has a late morning direct flight that leaves at 11:30 which might sound even better after a long late night of debauchery. However, you should keep in mind (especially if you're as safety conscious as the last 2 posters) that there is a good reason why there are so few domestic flights later in the day. Heavy rain and/or choppy air often becomes worse later in the day, particularly during the rainy season and weather has been at least as big a factor in aviation accidents in CR as any of the mechanical issues discussed above. At the very least, a greater chance of a cancelled flight or a bumpier ride exists on those later flights. If that hasn't put you off for a later departure, Sansa also has 2 later flights but they aren't direct so will take about 35-40 minutes longer to get to PJ. The first leaves around 11AM and goes through Drake Bay while the second leaves around 2:15PM and goes through Golfito.