Not sure what you are saying about the rubber-sealed Bearing, having a much lower RPM, as they will do more than your machine will ever run at, most that size will be good for around 7.500 to 12,000 RPM, depending on the brand/quality of the bearing, what are the dimensions of the top Bearing
The only difference is either having oil lubrication, or grease, Grease has a lower RPM but still more than you will ever need for your machine
An open bearing is not suitable for that position, as you have no way to lubricate it, & keep the junk out of it