One of the many mistakes I found that I made when I ordered my X-Carve was not taking advantage of Inventables pricing on a 1/4″ shaft, 2-flute HSS spiral upcut bit for $10. This s a pretty good price on this bit and ordering it separately from Inventables will incur a shipping fee of $7.50.
I looked far and wide and could not find a better deal than this. What I did find was a $20 Whiteside bit similar to the above bit above only in solid carbide. In fact the price has dropped to $17 after I bought it, making it a hell of a deal. The only caveat is that using a solid carbide bit in an untested machine may not be a great idea. Solid carbide is pretty brittle, and if your machine goes out of control or tries to take too big of a bite, it could snap the bit. HSS is going to be more forgiving because it is less brittle.
Solid Carbide does have it’s advantages though: it’s going to stay sharp longer. So if you are confident in your machine and have it dialed in, it should last longer than the HSS version.