i don't know where the '1210' number comes from, as that number is usually the height and width of the led chip-- that's where the 5050 comes from-- 5.0mm x 5.0mm....
1210 is a valid led size, but is way too small to be useful in the type of lighting you're interested in.. the correct term for that led size would be 3528 or plcc2...
generally speaking, 5050 chips have 3 chips within them, so they are brighter, but because the led is larger to accommodate all 3 chips, that means there's more spacing between each physical led. the 3528's would allow smaller spacing between led's, which will provide a more even output.
overall, i've used both in a few different custom projects, and i'd say it's a wash.. the 5050 looses some ground because of the larger physical size, and the 3528 gains some ground because of the smaller size...
personally, i prefer designing things around the 3528's because each chip only has a pair of contact pads to design for instead of 3 pairs..