Working in the motor shop, I've generally see motors with shielded bearings last longer in the field (bearing wise) than open bearings. This seems to be lack of attention to or no lubrication practices at all. Not sure if its lack of knowledge or just don't care. Sure I've seen the occasional motor come in with windings full of grease...but its usually a endbell cavity full of hard clay like substance that was once grease. I'd say if you have good knowledgeable folks that care, open would be best. If you have guys that don't care or are not attentive...shielded bearings might be the better option.
Just to illustrate, I have a customer that started using a Ultraprobe® 201 Grease Caddy from UESystems and the results have been great. This was at a place that "HAD" a "procedure" and PM schedule for their equipment that included lubrication. After they were tired of seeing lubrication flags on the vibration reports they bought one of these and assigned a machinist to go with me on my data collection routes. They were using those low volume grease guns and the first motor we came to took over 50 shots of grease with that thing before any hit the bearing. Not hard to imagine if you have a low volume grease gun and 15 inches of empty grease pipe to fill. The machinist was flipping out after the first 10, but I assured him that if something happened its on me. I was monitoring live peakvue data, and listening with my headphones on the 2130. we got his headphones set up prior to getting started as well. The look on his face with the grease finally made it to the bearing was priceless. He still follows me around but he doesn't hardly have to grease much these days as he has really become proficient and comfortable with that thing. He goes out at least once a month and checks on things and has gotten that caddy dialed in for the different equipment on that site.