Never worked with the 609 but I've been told the 906 is worth the upcharge.
Really, as I understand they're totally different mics. The i5 which I have and love is like a slightly brighter, slightly more scooped version of a SM57. Jacksonplayer liked it as a cab mic because you could use it in slightly darker positions than a 57 for a little more girth. It's similar but a great alternative flavor.
The 609/906 is if memory serves a larger diaphragm dynamic. It's usually used as a second mic in conjunction with a SM57/i5 rather than on its own, to fill out the low end a bit. It also, IIRC, has a huge sweet spot.
If you primarily want a new mic because you want to be able to close mic both sources at once, another 57 or an i5 might be the better option, but if not I'd be tempted to go with a 906 just because it's a more different tool to put in your arsenal.