Trifexis is Comfortis (spinosad) + Interceptor (milbemycin oxime). Comfortis is the flea med, the latter for heartworm. I used Trifexis for probably six or eight months while the Novartis plant was shut down and I couldn't get Interceptor. Now they don't make Interceptor by itself anymore, so I use Sentinel, which is Program (lufenuron) + Interceptor. Sentinel happens to be cheaper for me... that's pretty much the only reason I use it instead.
Iverhart, OTOH, is Ivermectin, plus meds for worms, no flea meds. And if you own one of the breeds that are sensitive to Ivermectin, you probably don't want to give it.
As to the ones that DO have flea meds - both Comfortis and Program have been around a long time. I've never actually had a flea "problem" - my boys did bring a few fleas back from the woods once and I spied a few dead flea bodies around the next day, but they were on Sentinel and Frontline both at the time, so... which killed the fleas? Not sure. Anecdotally I've heard the oral meds work better on fleas than topicals, but I can't say for sure from personal experience... I use the Frontline for ticks, not fleas.
I have shelties so no Ivermectin over here, but I don't have any strong feelings one way or another about either being "better" for heartworm than another. Personally, since I use a topical for ticks that also does treat for fleas, I would prefer to give an oral med that does NOT contain a flea med (although the meds are different.) But there's no option for that on the market right now for me. That might be the route you prefer to go though, through the Iverhart.