I'm a little confused...are you looking for a putter that will help with longer putts or a mid-range that will will be a good approach disc?
As far as putters go, I use 3. I have a DX Aviar because if I need a putter to sort of turn over or just wrap around an obstacle that's my go-to disc. I also use this an approach disc at times. I also have a Champion Aviar that refuses to turn over and will hold a straight line, this is what I use for any straight line putt and will give me the best chance for longer putts. I'm also working in a SSS Wizard into the mix because sometimes my Aviars are just too stiff that day and the wizard will grab chains all the time! :)
I bought a Nuke, but I personally can't throw the thing. I've had a little better luck with the Katana but not much. I don't have a cannon for an arm though, so if you do you'd probably have better luck than me.
I like to use my Discraft Stratus for long putts. Get a nice light weight version of it (pro D is my favorite for putting, but that's me) and go to town. It has significant glide and predictable fight paths.
I prefer an Innova XD for mid-range and long putts. It has a nice low profile and fits well in the hand plus it's dx plastic so it's inexpensive. I would probably go for max weight for windy days. It hold a straight line pretty well.
I say a Roc and a Buzzz. Two must have discs. As for helping on longer putts. I use a Classic Roc for those 100' to 125ish up shots/putts. Innova says it's a putt and approach disc, but I never thought of it as a putter. It's a great disc imo.
A Buzzz is a great disc but I prefer Rocs. Here is the thing, a casual golfer might prefer a Buzzz only because they are pretty much straight right out of the box. A Roc on the other hand is stable to over stable depending on weight, mold, etc. I carry at least two Rocs during practice rounds and four during tourney's; two 2006 USDGC Champion Rancho Rocs and two 11x Kc Pro Rocs. I am also thinking about the glow Dx. Roc but have not decided yet. Don't get me wrong, I do throw Discraft and Have Used the Wasp, Buzzz, and the Meteor...we're talking mids here but I keep coming back to the Roc. Take a Dx. Roc; When new it will fly like the first Z wasps' used to before the mold change. A Roc that is moderatly broken in will fly like a Buzzz(will left, right or straight depending ong the angle of release), also will tend to turn in a stiff breeze but always dependable. A Beat in Roc is like a Meteor; Straight to flippy at moderate speeds and very controllable. So basically my thoughts are that I like the Wasp, Buzzz, and Meteor but why have them in my bag when I can have pretty much the same mold that does different things. The key word is consistency. Now, about your putts. Stick with a putter. If the putter is hard to learn, learn it. The slower the disc, the lower the margine of error. I hope this helps. Good luck!!!
Honestly, I would suggest that you focus on learning to throw your putter further. One of the few bright spots in my otherwise mediocre game is my 50'-150' putter upshots. Past that range, I think that we can all agree that it's not really putting anymore.
P.S. I highly recommend a Nuke to anyone who can throw further than 300' consistently. I watched my best friend add 30'-50' to his throw simply by throwing a Nuke. The ESP ones break in fast, which is great for those of us who don't have a ton of power. Not exactly an easy disc to get the hang of though; I still struggle with my release.