There are several rifle-length piston kits out there, but I have not seen any fully-assembled rifle-length piston uppers. (I may have missed them, but...)
Like you, I wanted to have a rifle-length piston upper. Mostly, this was due to wanting to reduce the amount of time spent cleaning my rifle. But, admittedly there was also the "novelty" aspect of having a rifle-length piston-AR.
I've installed a couple of AR-15 piston kits before...an Adams Arms, and an Osprey Defense...both were carbine-length. Of the two, I like the Osprey Defense slightly better, as it's a more simple design.
That said, the Adams Arms is a very good piston system too. And, they do offer rifle-length piston kits as well.
Anyway, I decided to go with an Osprey Defense rifle-length Exo-coated piston kit for one of my builds last year. The installation is straightforward, as long as you feel comfortable removing your front sight base and you have the right tools for the job. However, I do recommend that someone has some "wrench time" around an AR-15 first, before tackling the job.
There are some nuances that you have to be aware of. For instance, for some reason, I was having fail-to-feed issues with the rifle while using standard GI 20-round mags. But, when I swapped them out for GI 30-round mags, it ran flawlessly.
Now, I think I've isolated this down to the follower not pushing-up the rounds hard enough. In fact, I noticed that the bolt carrier group was actually pushing the rear of the cartridge back down into the magazine. Again, this was only with the 20-rounders. Once I put 30-rounders in there, it ran flawlessly.
I have picked up some 20-round PMags to test them. But, I haven't gotten to try them in the rifle yet...too many other "hobby" projects have gotten in the way. Besides, when I ran 30's, the rifle was a beast...I did several mag dumps to verify this. :ar15:
Now to put it all in perspective...
Piston AR's are definitely something I like. That said, they can be finicky at times. And, you do have to watch for carrier tilt. Bottom line, when you add a piston to an AR-15, you're fundamentally changing the Eugene Stoner design. Moreover, you're adding MORE parts to your rifle. And, as it turns out, these parts are generally under extreme stress, considering the nature of what they are responsible for.
So, you're essentially adding some "complexity" to the design of the rifle. Does this mean that you should forgo a piston-AR? Well, no. But, you should consider the pros and cons of having a piston-AR. That's all I'm addressing here.
Lastly, as long as you don't mind cleaning your AR-15, the direct impingement (DI) system will serve you well. And, unless you're shooting thousands of rounds without lubricating your firearms, a DI rifle will be reliable. At least, that has been my experience with my DI AR-15's.
So, what does this leave you with? Simple...it's a personal preference. If you want a piston-AR, I say, "Go for it!" You'll definitely save some time cleaning after your range sessions. Plus, there's just something cool about them.
Hope this helps.