Dude:
I would be tempted to go with the RGBs, simply because I think it would be cool to light up the buttons by game, in the same color they were in the arcade. If you are not doing that, then I don't see a huge point during gameplay for RGBs. That would only really be cool during the attract mode IMHO.
Something to think about before adding a 4-way:
Do you like playing 4-way games with the 49-way joystick set to 4-way mode? If the answer is yes, then there is NO reason to add a dedicated 4-way. If the answer is no, then you should add a dedicated 4-way because there really is no better way to play 4-way games. I am so partial to the feel of the original Pacman 4-way joysticks that I restored a whole Pacman Cocktail with original PCB and joysticks because I didn't like the way any other joystick felt in Pacman in MAME.
The next question you need to ask if you are going to use a dedicated 4-way joystick is: Do you like the way the 49-ways feel in 8-way games? If the answer is yes, do you like the way they feel more than a dedicated 8-way joystick? If the answer to either of those questions is NO, then maybe you shouldn't be using the 49-ways at all. You might be happier with a dedicated 8-way that you love and a dedicated 4-way that you love.
Now, if you really like the feel of the 49-way joysticks, then just use them. For games like Assault where you need dual 4-ways, it would be nice to be able to set all the 49-ways to 4-way mode. However, for games like Assault and Tron you need some kind of triggerstick or thumbswitch joystick. Are you into those games as well?
Let's talk out all you want to play, what your favorite games are, and what games you will feel limited against, and we can come up with as good a solution as possible. For me, the only solution I'm going to be happy with is a modular system. However, that eliminates artwork and LED lighting, so everything's a compromise.
BTW, I know I'm not telling you anything you don't already know, but sometimes it's nice to bounce ideas off of somebody. I'm going to be bouncing some MameWAH stuff off of you here before long.