Tough question, but a good one.
To begin, putting salt licks and machans aside (no judgement being passed here), both hunts usually involve extended periods of tracking. Tracking puts the hunt in hunting for these two species, which can be quite physical and mentally taxing.
Some bongo hunts take place in the bushy areas of streams that cut through savannahs like what's found in Eastern C.A.R. Lord Derby co-exist in the same area, but the thin slivers of riverine bush offer respite for the bongo. Other bongo hunts take place in the forest where the bush is so thick that you'll strain your eyes to see 5-10 yards. As noted previously, a forest safari like those typically conducted in Cameroon, is quite magical. Pygmy trackers will follow the tracks EXACTLY where they go meaning that they will follow them under and over everything. You will crawl. You will duck. You will try to avoid columns of voracious ants. The Pygmy trackers are accompanied by a small team of dogs which they will release when the gap between the hunting team and bongo shrinks appropriately.
All hell breaks loose. The shot will be close. This is one place for good iron sights or a scope in the lowest possible power setting. One advantage commonly attributed to this type of hunting is that bongo may hold up long enough to judge the animal. Immature animals can be spared. The forest safari can also offer forest sitatunga (very special in my book), dwarf forest buffalo (another magnificent animal), red river hog, giant forest hog, and some duikers. They're all desirable.
If it's true that you kill an elephant with your feet, the same can be said for Lord Derby when tracking them on the savannahs. Have the wind change or make a bad move and the herd will gap you quicker than anything. And they can cover a lot of ground when they are alarmed. Depending on the time of day, consider picking up the track the next day or trying to find tracks elsewhere. It's hunting in a very pure form. I was on a safari that involved following the tracks of a herd bull for 3 hours, and yet we were still only 200 meters from where we parked the Land Cruiser. It had been milling about with about 15 other eland and we finally saw where it had broken out from where they had been feeding. The chance to take a Western Roan, Giant Forest Hog Savannah Buffalo. and other game makes a savannah safari special in its own right.
Both species have attributes and qualities that suggest the final choice may come down to the type of hunting that you prefer. Currently, there are no import issues, but the war on hunting always makes the future uncertain. Good luck.
