It is really hard to compare cruiselines. But I'll try. First you ask for an unbiased opinion and I wonder if I can do that.
So anyways here's what I say
First so much depends on the cruiser's expectations. So with that in mind I'll tell you that we have cruised
RCL X and HAL. We found them all to be just fine thank you very much. We started on RCL graduated to X and threw in a HAL here and there. Since you asked on the X forum don't be too suprised that since I hang out here it's my favorite.
Just a little better at most everything I guess. Nothing overwhelming but service, dining and the M class ships with lots of room on them would clinch it. They have a "Happy" crew and that makes a big difference.
Really at xmas with a lot of family stuff happening is a tough time of year to cruise. Not only pax are on a special holiday but the crew is working away from home and "Stuff" happens, or so often doesn't happen.
Fewer public announcements is nice. It means quiet but if your somebody who likes to take part in stuff it can mean a lot more studying the daily program. We often rely on a certain friend who has us going to the correct place at the right time..When she's not there we miss her.
You can see that it's the little things that make the difference and I think X has them. But I don't think you will find a great difference at all and wouldn't vary from my budget a lot expecting to gain the same difference and experience.
Here's where we've been an we started at about your ages
RCL Song of America & Splendour of the Sea's
X Zenith Horizon Infinity
Oh did mention the martini bar 
Hal Veendam Oosterdam
