The main difference is speed. The JV150 has 8 ink slots, so 4 colours uses 2 carts for each colour, so prints faster (less passes).
With 6 colours you get the standard CMYK, + lC and lM. You can also add orange and light black which really increases the colour range.
CMYK is probably good enough for garment, but will probably be way faster than you need ( good for banner printing though). 8 colour will give you a lot of options if you want to do some sign/graphic printing, but will be a bit slower.
You need to take a good look at your business to see what would be best. If you don't currently offer signs and banners, it is probably something you could add.
Can you stretch to the CJV 150 - not much more money, and it never hurts to have an extra cutter.