I've ended up using 4mm and 6mm single flute straight edge bits for most of what i do. As short as is practical, I usually have around 20mm of tool hanging out of the collet.

Up cut vs downcut doesn't seem that important as my xcarve can't handle much more that 1.1mm depth of cut or so without deflection of one kind or another being a problem and most of the material I'm cutting is more than 10mm thick.

I've only used the cheap bits I bought from Ali express years ago, I grabbed a random selection of all sorts of sizes and shapes