I had almost the same boat (mine's a 548 and not a 648), 25hp 4 stroke (2000 model), but short shaft. I had the same issue. Hard to tilt it. There's an easy way. Kick it into reverse while it's idling and you can tilt it a little easier. That's not always convenient, though.
I had a hair brained idea this past summer that if I could sell my '00 model for any appreciable amount of money, I'd put the money toward a new Yamaha 25. So I stuck it on CL for a high price and wouldn't ya know it, guy emails me about 10 minutes later and wants it for the asking price. I figured he was another blow-hard, but no, he actually showed up that evening from about 175 miles away and bought it on the spot. 'Course I sweetened it up a little by giving him some spare parts that I had amassed over the years doing dealer work. CDI, prop shaft, spare carburetor, and a spare prop. But the problem was, the dealer was out of new 25's and it was going to be a week or two before another came in. That was the longest 6 day wait I've ever had. I had to resort to putting the old 3 cylinder 25 back on the transom. It's fast but once you've had a 4 stroke, you feel weird going back to a 2 stroke. About as awkward as going from 2 stroke to 4 stroke.
Anyway, what I can tell you about the new Yamaha is that you'll go about 2 mph faster. It starts better. The throttle is MUCH more linear. The old 4 stroke motor, once you get to about 3/4 throttle, if you go from 3/4 to full, it doesn't really pick up much. With the EFI on the newer one, it is a lot more controllable. Full throttle is full throttle. 3/4 is 3/4. There is a difference between 3/4 and full; on my rig about 4 mph give or take. The first thing you'll notice is a little shake at idle. It is not bad but it is noticeable. I have about 10 hours on mine now and a lot of the shake has subsided; so keep that in mind. But really the BIGGEST difference is that tilting it is like tilting an old 2 stroke 25hp twin carb Yamaha. It's really that easy. Even for my old working man's back that hurts mostly all the time, I can tilt it up one handed. The tilt lock takes some getting used to, being that it's in the front of the motor instead of the side. And the starter button is kind of buried on the lower cowling too, not really buried but not quite as easy to get to as your older one. You get used to it quickly. Now the technical. Yamaha's EFI is super nice. The lower unit is mostly the same as the old 2 stroke twin carb, with minor differences in the shape of the exhaust, and a better paint process. The prop shaft is the same part number as your current 25 as well as the old 2 stroke. Water pump parts are the same as the old 2 stroke, but your current 4 stroke is different than the new 4 stroke water pump. The cowling lock is easier to use. The motor is considerably quieter under way as well in comparison to the older 4 stroke and a lot of that is due to the elimination of the counterweight "slug" in the older 4 strokes. The new 25hp "pee hole" doesn't pee quite as hard as the older ones which I find odd since Yamaha's known to have a pretty stout stream of pee water. But I've run about a dozen of them and they're all the same (brand new). Water pressure is normal, they just don't bypass as much water as the older motors did. Also, the entire motor's paint is a little nicer on the newer ones than the old ones were; more glossy. That may not matter to a duck hunter, though. Most of them will spray paint them camo the day they get 'em home.
How does it compare to the Tohatsu? It's (the yamaha) more refined. The Yamaha lower unit doesn't whine. It's silent. The engine is quieter under 1/2+ throttle than the Tohatsu. The idle quality is a little better on the Yamaha, though the Tohatsu runs slightly smoother (though not a lot). The yamaha is about 1 mph faster top speed too, with no appreciable out-of-the-hole performance difference.
Suzuki? I like the idle quality and that's it. It felt a little sluggish from holeshot, was about 2 mph slower top end, on the same hull, and it was right at 6100 RPM on the tiny tach. The shifter felt weird when shifting. I had a little trouble with it, but it could've been that I ain't used to it. I just wasn't happy with the suzuki 25....and I WANTED it, but glad I didn't buy one now.
I had some vids on my phone from this past summer's use of the new motor, I'll see if I can't put 'em on youtub.e.