Rotterdam is significantly larger than Tahitian Princess but still relatively small by todays cruiseship standards. They were built just two years apart so they're close to the same age. Holland is generally a little older crowd although on a World cruise both will be predominantly senior, the only age category that has that much time for such a cruise is retirees.
Both ships get glowing reviews. On a site I use frequently the Tahitian gets 4 stars and the Rotterdam gets 5 but the star ratings can be misleading. On the site I'm referring to they are a compilation of a number of rating systems by different sources. It's heavily weighted by feedback from actual passenger surveys upon their return so it's not just the cruising press and a handful of writers opinions.
I think either ship would be a great experience. If you want early dining you'll often have to book a year or more in advance as it is the first to fill and fills quickly. With the personal choice dining as an option now on both lines it won't be the big problem it was in the past.