YAMAHA's are pretty proven, but they have NEVER impressed me with there power. There new 4 strokes are quiet though, everytime I go out with my buddy I keep telling him the engine has stalled. SUZUKI's are a good engine as well, but not the greatest dealer support network. BRP is REALLY pushing the ETEC as the greatest thing, but it is still the same basic technology that the FICHT was back in 98. If your after ULTIMATE milage in the 150-175HP range, then I would suggest the OPTIMAX as the #1 canidate. Tests have proven that the 150 OPTI is pretty much KING of economy on fuel and oil(excluding the ETEC set up for XD100). The YAMAHA 150 4 stroke would be a good match I would think as well as the 150 HONDA.
The MAIN question with buying new is warranty and a GOOD dealer. Remember that these new engines can be REALLY good and reliable, but on a direct injected engine a couple of injectors or an EMM will cost almost as much as the engine is worth. Look around and see how many OPTI's you find needing air pumps and injectors for sale because those parts cost more than they can buy a replcement for. Technology is great, but it is a killer as well. You can roll the dice and find a late model used HPDI YAMAHA, OPTIMAX or FICHT and hope for the best. Me personally if you don't use the boat ALOT, then just get a reliable old 2 stroke. For what you save on gas with a new engine, you will spend on the initial cost.
The V4 JOHNNY-RUDES are pretty D@MN reliable and powerful, as well as not being horrible on gas. The 2.0L and 2.4L MERCS can be had for decent money and are decent on fuel as well as a LARGE availability of interchangeable parts. The old tried and true 150 YAMAHA 2 stroke is tough to beat for reliability and durability, and is one of the better 2 strokes on fuel.
Decide what you want to spend, and go from there. All of the new engines are good, just make sure to get a GOOD warranty if you decide to go new. Your current figure of 1.7 MPG isn't that far off from some of the better numbers here as 2 MPG is regarded pretty good economy for a V-20 with a 2 stroke. You may even be able to improve your current set-up to 2 MPG by getting a better designed prop. You would be SHOCKED to see how much extra fuel one nick on your prop will make you use.
ON EDIT: I see you currently have a 150 MERC, so you should have a 2.0L. That is actually a pretty fuel miserly old school 2 stroke. A little time with boat set-up and propping may make a BIG difference in your economy and performance. I have a 150 MERC on a bass boat and it only used 4 GPH at cruise, even on a V-20 I would expect it to stay around 10 or so fopr fuel burn at cruise.