A "regular" BiPAP is portable w/ a battery pack and DC cord. If you are not venting, a Trilogy is a large AVAPS (volume controlled) BiPAP equivalent.
Unlike the Trilogy, some BiPAP models can go in your lap for travel. Our Z1 CPAP goes over the wheelchair back/lift cradle via lanyard. A Trilogy must go on the back of the wheelchair in its bag when out, which can be a problem if you don't have the depth available in elevators, restaurants, venues, vehicles (in our small apt, even aggressive tilting creates space issues). The Trilogy is 12 lb so moving it w/o wheels is nontrivial. I don't think any one machine can be generalized as "the best" for all.
I believe some people here, like Paulette, also find a cheaper BiPAP more appropriate for shower use, transfers and the like. We have 3 PAPs in use (bedroom, living room, transfers). None is a Trilogy. My husband's FVC is 20-something, maybe less. As you say, if you plan to vent, it may be easier to get the Trilogy from the beginning, but no machine is perfect.
Trilogies are available for sale used but, depending on life expectancy, rental is likely cheaper. As for other BiPAPs, they are as well (available both new and used, depending on the model; I recommend SecondWind), and the insurance rental proposition is a total ripoff financially, as well as disincenting your own optimization of settings/masks, so I would buy unless the user's future is numbered in months. The same goes for a suction machine, even more so. For the CoughAssist, I'd check the numbers both ways.