I have both, and have turned a few large bowl with them. Here’s a pic of a medium-sized applewood bowl with both accessories in action.
If I had to choose between the two, I’d defintely choose the speed reducer. The UTR is nice, but a large unbalanced blank spinning at 700 RPM on a lightweight machine is downright dangerous. And even if it weren't, it would still try to walk the whole darn machine to the other side of your woodshop.
I also use the speed reducer for large Forstner bits. It clamps tightly to the quill, so loosening the way-tube clamp for drilling really isn’t a problem, as the load is almost purely axial. And the bracket still slides along the way tubes, providing an additional measure of support.
When space and budget permit permit, however, my next Shopsmith lathe upgrade will be an 800 lb. bowl lathe! 😃
