I have both and I tend to think the stainless is better for a hobbyist since you can leave it outside, accidentally or not, and it’ll be fine when you find it a few days later on your nursery bench. Although, one of my pairs of carbon scissors are my go-to for a lot of easy trimming. Super super sharp, but I’m thinking that’s just on the manufacturer and a stainless could be the same. I’m probably too lazy to regularly sharpen my bonsai tools which would be the plus for carbon, and I’ve already had to sand off rust, so I’ll be sticking with stainless for the most part.
like @caffeinated said, I have and use both. I do try and keep my tools inside and dry, but I don’t always and they sit on the covered patio so I too fight surface rust on occasion. I will say that I find myself picking the tool that I know cut’s best regardless. My favorite pruning shear is stainless, root shear carbon, flush cutter carbon, etc.
Steel is harder, rust proof, the edge sharpness is more durable staying sharp longer before needing maintenance.
Carbon is softer, a feature that, despite seeming a minus, allows to create an even sharper edge, but it is less durable, requiring more frequent maintenance.
To cut it short, carbon is much more performing and many masters prefer it. It requires more maintenance to keep the edge at sharpness levels that cannot be reached by steel. The steel is more robust and durable, but it sacrifices a bit of sharpness (i.e. as @caffeinated writes, the stainless is better for a hobbyist). Remember when speaking about edges… metal softness enables sharpness. Because anyone has different feelings about the tools, I would say that you should chose what gives you the better personal experience while using.
I voted stainless. I prefer the aesthetic of carbon and as mentioned it’s supposed to be possible to achieve a sharper edge. But in practice, stainless is more convenient due to to the low maintenance.
I prefer carbon to stainless in the long term. Yeah it may take a little longer to dull stainless tools but sharpening them is much more time consuming. Carbon steel tools can be quickly and easily sharpened.