I wouldn't dismiss the Zoner dessie so quickly. It's nimble and has excellent firepower - not to mention, the roleplay can be (almost) whatever you want. You don't even have to go near the corsairs if you don't want to.
EDIT: Also, in 4.86 it's getting an excellent remodel by Jinx.
Be kinder than necessary, because everyone you meet is fighting some kind of battle.
J.M. Barrie
Hehe, you got the same problem I got. I'm thinking about what I should get - either Bret Cruiser or Kusari heavy GB (or even BS).
But back to your question.
This question came up some time ago from another person I think.
In my opinion the Rheinland Cruiser is the heaviest and the best cruiser in the game. But that's just how I feel about it.
I know some smaller cruisers might be better but ... isn't interesting here anyway.
The Bretonian cruiser has some weird firingarchs while the Rheinland cruiser can fire all it's turrets to the front (if you aim a bit to the left/right and keep aiming there) and all but two to the rear. The sides are also covered by a good amount of turrets.
I haven't played the bretonian cruiser more than just on the singleplayer to test it a bit, but as far as I can remember the turret firingarchs weren't as good as those of the Rheinland cruiser. So iPvP the Rheinland Cruiser is superior. (Not to mention it got one turret more and more armor, but as far as I know it's slower.) Another thing is, the Rheinland cruiser is a bit harder to hit from the front, if I'm correct. Further things about PvP - their enemies: At the moment Rheinland is facing small cruisers, but not as small and op as the actual Kusari cruisers. Their enemy's cruiseres are weak but agile, the fights are not that much balanced, Rheinland is a bit superior if the captain of the Libby cruiser isn't the good with handling his ship. If he is, he might destroy the Rheinland cruiser, but chances are still, he gets beaten. - While in the Bret/Kusari war the Kusari Cruiser seems to be much more powerfull, though it isn't. It still beats the bigger and much less agile Bret cruiser with far less problems than a Libby cruiser fighting a Rheinland Cruiser, because of the Bret Cruisers turret firingarchs. But ... most enemies you will encounter will be Bombers anyway, and ... those are a real pain in the behind for both cruisers. They are big targets.
The situation will change with 4.86 for Bretonia and they will start fighting much bigger cruisers when Gallia attacks them. Once that happens, the cruiser fights will become more balanced Imo.
To sum the PvP situation at the moment up:
Rheinland is fighting and will always be fighting the more agile ships of Liberty. But the cruiser is extremly powerfull and a good supportcraft for their heavy battleships. Still they have huge problems with enemy bombers and other small and agile ships. In 1on1 it can beat every other cruiser.
Bretonia is fighting very agile and very small enemy cruisers at the moment. Their cruiser seems to be a bit less powerfull than the Rheinland cruiser and it's turrets aren't that good. As support it might be good against the heavy Kusari BS, but overall I don't think it's as usefull as the Rheinland Cruiser. Like the Rheinland Cruiser you will have problems with small enemies.
That's my non-objective opinion. :)
About RP on those ships...
Well, both got good RP situations, but I think Bretonia might be a bit more interesting, with their Dublin and a losing war on their hands, which will even get more horrible than it is now. Fighting on every side and defending your country against enemies which are far superior to you. You will become insane and pressure your crew to the end, fight till death to defend millions and millions of lifes, while in Rheinland the war is more about ... well ... fighting? It seems to be just because they decided to have war. While Bretonia fighting Kusari seems to be personal stuff and Gallia vs Sirius is just the same...So it's way more intense. Not to mention you also got the corsairs sitting "beneath" you, to the south.
It's your choice. But I can tell you, you will be frutrated to find out, that most enemies will be bombers, who won't RP much and just come to destroy a nice little...eh...huge cruiser, which they can do with ease.
Please correct me, if I'm wrong, but keep in mind, that this is, what I experienced.
Edit:
Oh, and about a Zoner cruiser...
Don't.
Just don't.
iRP they make no sense, I got that after I had one myself.
Only if you RP it as a science/research ship, with no heavy weapons on it, it could make sense. Zoners aren't a militaristic ehm...group. I hear it's getting remodeled? If I like the new looks of it, I might buy one for some science RP. But again - if there's even a fight, you won't be happy with the side you choose to be on. Neither will you face fair enemies - mostly bombers, nor will you face fair encounters with fair 1vs1 cruiser fights. So if you are buying it for the PvP - don't. You are way better of with a Bret or Rheinland Cruiser.