Hello,
Just saw this. I recently purchased an HDV-T16 and I've previously ridden both Cuda 550s and 650s as well as owned a Sierra a few years ago.
I've had the T16 in the water less than 10 times (from both boat and shore), but my initial observations is that is it is a REALLY solid machine.
Some basic observations you've likely met before:
Cuda has more burntime in its default configuration.
Cuda is longer/more stable.
Cuda has more adjustible speed options.
With respect to the Halcyon, burntime is far in excess of an hour even with steel doubles and drysuit so, burntime is less of an issue depending on what your buddies are using in addition to what your dive plan is. This is quite different than, say, if you had a Sierra and your buddies had Cudas, in which case, it would be a very short dive indeed, at least for you.
As for stability, my experience so far is that the Halcyon meets a sort of minimum length standard that passes it into the realm of stability needed when on the trigger for long periods of time. Assuming a post-fit Lithium battery purchase for your Halcyon, my assumptions are that I would be just as comfortable on the T16 for a 2 hour ride as I would on the Cuda. Actually, moreso, for the next reason.
While the Cuda gives its speed variation with gears and offers a much wider span of speeds, the Halcyon also offers speed control via a dial you can use on the fly that is just above the shroud. The N-handle on the Halcyon is, in my opinion, far more comfortable for long trigger times than the trigger type T-handle found on the Cuda. In addition, it features a cruise-control lock which I'm already enjoying which frees up even the one or two fingers you have on the trigger. In terms of exposure, both are about as "exposed" from off of the body/shroud, with the Cuda perhaps being a little bit more exposed than the Halcyon. I don't find this much of an issue unless you're in a cave with one trailing off your butt d-ring since, if done properly, it will be out of the slipstream and not exposed to lines or other obstacles.
What propels the Halcyon ahead of the Cuda, in my opinion, are the subtle design/engineering elements. Firstly, battery removal and insertion is FAR more enjoyable with the Suex-built T16. Don't underestimate this, since it was a large source of my frustration when I owned the Sierra. The battery on the T16 is kept in place by lowering onto the central rod that passes through the scooter body until it meets the plate at the bottom, and it is kept from rotating about the rod by a second shorter rod that also fits into the battery which is also at the bottom of the plate. Finally, a retaining cap is merely screwed on top of it to prevent it from sliding in the opposite direction (towards the nose). There are also a trim and weight plate provided that can be inserted below the battery on the same rod if you intend to use it in salt water. Nothing to be measured as in the weight pouches on the Cuda that are required to be filled with shot weights. I don't know if Dive Xtras changed the numbers stated that were supplied with the Sierra, but when filled to their recommendation for salt water, mine was about 2 lbs positive, and I found that the weight pouches were nearly full so I couldn't add much more shot weight to them to trim it down. I eventually added more velcro to the nose and shot weight pouches I made by hand, but then the trim of the scooter was off. Anyway, not to belabor the point, but I had none of these issues with the T16. It is about a half a pound positive in salt water with the plate inserted and nearly perfectly level at depths above 60 ft. Below a 100', it looks to be completely neutral.
Secondly, the T16 features some extra features like the emegency bypass which allows connecting the battery directly to the engine in an emergency to ensure return to entry point even if something in the trigger mechanism or electronics were to fail. I don't plan on using this of course, but it's nice knowing it's there. And, the battery cutoff alarm is nice too.
Thirdly and finally, I like the little touches like the way the delrin and aluminum are used together to form the dpv. The sound of the engine (which you notice over time but is also important to be able to hear your buddies) is a bit more noticeable on the T16 but in a soothing way. It's not the high-pitch whirring noise of the Cuda. The charger is also nicer and performs automatic switching between countries that use 110v and 230v, keeping user error from blowing a fuse. And, of course, the T16 is about 10lbs lighter in weight.
I'll end this for now by saying, that while I love my Halcyon and hope to purchase another to have on hand in the coming months, if I hadn't gone with this (or the similar Suex XJOY14), I would have bought a Cuda 650 (used or otherwise) since that's what most of my buddies have had for the past couple of years.