My final conclusions in trying to answer this question can be summarized like this:
- The DR does not require any special visa to stay essentially forever, Colombia does. In Colombia you have to continually worry about when your visa expires: in the Dominican Republic you do not.
- Colombia is at least slightly more dangerous than the DR; more senseless crimes and serious murders.
- The DR is closer to the US.
- The DR has a longer history of protecting expats and their property when they decide to buy and live long-term here.
It is important to understand that on the whole I actually enjoy Colombia more. However, the first reason above (regarding lots of visa and red tape requirement in Colombia) was the main reason I think the Dominican Republic is better than Colombia for long-term living as an expatriate.