IMO carbon for a stem is a non issue. Always liked the Ritchey aluminium stems, especially the WCS line, but they now have an adjustable model which seems interesting:
I saw the adjustable stem at Performance the other day. The weight would be my only objection. I believe it’s more than 100 grams heavier. Maybe that isn’t a huge deal but how often are you going to be adjusting the stem?
There’s one more thing you need to know before shopping…clamp size. Make sure your next stem has a 26mm bar clamp to fit your Visions. Don’t buy anything marked “oversize”…these have 31.8mm clamp for “oversize” bars.
0 degrees doesn’t seem to be too common through search. What will a ±6 degrees rise do? what difference will it make.
It means a couple of cms +/- in bar height, assuming you don’t do anything with the spacers. If you want exactly the same par height, you can either add a spacer (assuming there is a spacer on top of the bar) or flip the stem so it has a +6 degree rise and remove 1 cm of spacers. That should put you at about the same bar height.
O degree stems are getting rarer these days. That’s why I posted the Ritchey adjustable stem. Ironically, I just sold a rare 0 degree Ritchey stem on ebay not too long ago.