Thanks for all the replies. Valid points on either side, that is, to go with a single cylinder 4, 5, or 6, or go with a twin cylinder 8 or 9.8.
If I knew someone that had a 8 or 9.8 four stroke I'd test it out and primarily see how slow I could get it to troll. Several people have commented on here that neither an 8 or 9.8 will troll as slow as 4-6hp and that is pretty much a deal breaker. Also considering an 8 or 9.8 is another 30ish lbs to carry on my knees (I have a canopy) in and out of my truck each time I use the boat I think as I age the extra weight would start to become a nuisance.
Yeah I could just troll with an electric but I find having two motors on the transom, fuel tank + battery in the boat, and so on too much of a pain cost/benifet wise. On smaller lakes (esp those electric motor only) we just pack the electric (duh). On lakes that are "too big" to cruise around and troll with just the electric, I prefer to just bring a gas motor and use that for both cruising and trolling.
Sometimes I/we slow troll a fly to prospect out a new lake and find where the fish are, other times we slow troll a flat fish or wherever. Being able to troll nice and slow 1mph or so seems to be the key. The 5hp Merc I borrowed off the father in law trolled about 1mph at lowest throttle and no wind. Pefect. Yes the electric will troll even slower but I find generally I'll increase the throttle until we are moving at 1mph or so. For sockeye (Babine Lake) we we typically doing about 1.8mph which killed the battery in 3-4 hrs with no wind. Not juice for a days fishing.
I havent bought anything yet but am likely going to go with a 5hp Merc. I have used this same motor on my boat and liked it a lot. A few different forums I read (not this site) said that the 6hp idles a bit faster that the 5 and therefore trolls a bit faster. I dont think the 6 will do anything the 5 cant except move my boat a bit faster at WOT. Leaning towards the 5hp.