Personally, I prefer around 4000-4500K with a decent "CRI" (Color Rendering Index). Seems its a personal thing - lots of people prefer 5000K+ and some even as "warm" as 3000K which is close to a regular old tungsten filament bulb. Some of the cheaper LED lamps have a pretty spikey spectrum, which may or may not bother you. Problem is trying to match things like paint colors. If the maker doesn't spec the CRI, chances are it's not all that good.
Another difference between the tubes is the distribution of LEDS inside. Some can be rotated to get max light, some are fixed but do well. Others will show a sort of Xmas tree effect with individual LEDS showing through even a diffuser.
My lamps are in 3 banks of 4 fixtures - each fixture with four 4' lamps. Unless I'm doing really fiddly work, only half the lamps need to be on to have plenty of light. I'd try a 4000K and a 5000K bulb and see what you like. As far as brightness goes, you're almost sure to get as much or more light.
While it's possible for some LEDS to pop in without changing the ballast, as you probably know it makes much more sense in terms of energy and reliability to bypass the old ballast.