First of all, there is no general consensus. Some of the best shots I've seen were not even taken on full-frame bodies. I am talking about XTi, 40D, and D200 shots. You might want to find out if apprenticing with the D300 you already have makes sense.
A full frame camera will give you a wider angle and more megapixels; but going above 12 megapixels is often irrelevant unless you require prints larger than a two-page spread - 11"x17". You can obtain a wider angle on a crop sensor with a shorter focal length lens anyway.
The D300 is not holding you back, and neither would a 50D. Heck an XTi or D60 takes great shots and is lightweight; but just because they are smaller they are regarded as less pro by newbies.
Canon and Nikon have been leap-frogging each other in technology for ages. The only things I can say are that all the cameras you have named are great and maybe even overkill.
Canon standard and neutral presets produce colours that are accurate and representative of 'real' colours. There are presets that can exaggerate colours in JPEG output, just the same way that a Nikon does by default. Slight over-saturation and a little extra sharpening on nearly every shot is usually pleasing to the eye. Nikons also offer additional features above Canon cameras in terms of the ability to make on-camera edits to shots.
My only qualm with this is, why would I ever want to edit a photo on a tiny LCD screen..... and why if I were being professional, would I ever shoot in JPEG mode? Personally, I do all my editing through Lightroom or Adobe RAW Importer for Photoshop, on a screen big enough to see what I'm actually doing.
Canon lenses are great, and there are a lot more different lenses available through Canon and the third parties than there are for Nikon; but the lenses you need for wedding portraits are available on both systems.
So we are effectively back to even.
I would suggest that you do the following:
1) Favour Nikon because you already have one and can keep the D300 as a backup
2) Go to the store to hold and play with the other cameras you fancy... one of the most important factors will be how fast and comfortable the controls are to access in a hurry between shots
3) Do not mix systems with a Canon backup and Nikon main... because you want to be able to swap extensions if one body breaks down or one battery dies (and it is ideal to have two bodies with you so that you don't have to do lens changes)
4) Consider what your employer is using... because you'll probably want to be able to trigger his/her off-camera flashes - in portraiture, lighting is very important
5) Ignore the very brief, partisan, irrational answers others have given you and trust yourself
6) Consider your budget and maybe grow with time (you can invest in lenses now and get the upgraded body later, cheaper). If you are going to carry two camera bodies, make sure you have one long zoomer (maybe 75-300 with IS) and a medium-tele prime (85mm or 135mm with f/2.8 or wider)
7) Budget for tripod, memory card, external flash, diffuser, remote flash trigger, extra battery, polariser (if you plan to do any outdoor shots)
8) Read more than one review on an item
9) 24+ megapixels would be great if I needed to print portraits off of people in full scale (cardboard cutouts?).
I shoot mostly with a 450D/XSi (with 50mm f/1.4, 100mm f/2.8 macro, 24-105mm f/4L IS, speedlite 580 EXII with gary fong lightsphere II diffuser, off-camera slave speedlite 430 EX, tripod, and remote shutter cable).