The comments so far lead me to believe we are mainly comparing the latest 2 stroke technology versus 4-stroke. Keep in mind they still make the older style 2-strokes - the ones that use vaccum fuel feeds instead of direct injection.
I'll continue the comparison between direct injection (each manf. has it's own proprietary technology - but all boil down to this group) versus four stroke. Four strokes are basically a car engine re-tooled into a compact outboard package.
First Economy: Three areas - consumables (gas, oil, etc), routine maintenance, initial cost. For comparison purposes I'm using a Yamaha 200 HP engines, the F200 (four Stroke) and the Z200 (two stoke w/direct injection technology). The engines were both tested on the same boat (a 23' CC made by Sailfish) under similar conditions (weighting, weather, etc).
2007 MSRP as rigged on the boat - difference in price $3,970 - the Z200 being cheaper.
All gas/oil consumption is based on Yamaha's published life cycle figures which are 200 hours a year over a 10 year period. Naturally everyones usage and retention of the equipment will vary but we are using the same numbers for both, thus apples to apples. For this analysis I'm using 3500 RPM for both engines - that seems a fair average (based on percentage of efficiency for usage combined - if you want the details I'll explain otherwise you can just take my word for it).
@3500 RPM the Z200 uses 7.2 GPH versus the F200 at 6.8 So over the 200 hours of usage in a year, (Yamaha's life cycle assumptions) @ $3.00 per gallon the 2 stroke will use $240 more in gas. Over the ten year life cycle that is $2,400.
Comparing costs, So far we have:
F200 = (3,970)
Z200 = (2,400)
_____________
F200 = (1,570)
Now for oil
Z200 = 50:1 ratio so 1440 gallons per year would use 28.8 gallons of oil - cost = $668.16 Ten years = $6,681.60
F200 = 2 oil changes (every 100 hours) at $34.94 each (includes 5 quarts of oil and filter). Ten years = $349.40
Our Summary so far F200 = ($1,570)
minus the difference in cost of oil: Z200 = (6,331.60)
Total = Z200 cost $4761.60 more over the 10 year life cycle or $476.16 per year.
Some will argue they don't run their boats 200 hours a year - some run it more. But these figures give you an idea on how to compare true costs.
As to maintenance - other than the oil change, the recommended routine maintenance lists based on hours and items are almost identical. I'm not sure where the timing belt change comes from - it's not on the list. Both engines use spark plugs and they need to be changed at the same frequency. So maintenance cost seems to be similar. (granted if you pay someone to change your oil it will use up some of that $476 per year savings on the F200. Most shops around here charge $75 per hour and an oil change is 1/2 hour so it woul be $80 per year (doesn't count the lower unit lube or other items which are the same for both engines).
Now for Performance - I agree the nod goes to the two stroke - it weighs 55 pounds less and goes faster at the same RPM (about 10% - but of course uses more gas). The time to plane for the two stroke was 3.57 versus 3.80 seconds for the four stroke.
Emissions testing - the 4 stroke gets 3 stars (US Government system), the 2 stroke 2 stars: edge = 4 Stroke
I couldn't find the DB ratings for the engines but I have seen them some where before and the 4 stroke is quieter at all RPMs.
So as you can see the cost, efficiency & performance are similar with each getting a nod over the other in certain areas. So it really comes down to preference.
Oh on a side note - the 4 stroke is easier to work on according to my brother (a certified mechanic and engineer at Mack Truck). He says if you know car engines you can work on a four stroke. Not true with the two stroke - you need a marine mechanic for that he says as the technology is very different. Just his opinion of course.
