You should also consider other factors:

1. If you are MFUing WT+ to CW, then the onboard expenrience will be far superior on the UD of a 747 going to MIA, than the dormitory of a 777 going to DFW.

2. Same day connections may only be available through MIA - check the schedules for the dats you want to travel.

3. Connections through MIA are bad, but not horrific, although general advice seems to be to avoid weekend afternoons if possible. If you allow 2-3 hours for a MIA connection you should be fine. Just think you are in a latin 3rd-world country and your expecations will be set accordingly. DFW is peaceful and clean and not even to be compared with MIA.

4. If you are buying the onward flight, you will probably find it is much cheaper in J from MIA than DFW.

So, IMO the advantages of the UD, the convenient connections, and the cost savings on the onwAArd flight more than outweigh the negatives of transitting through MIA.

You will need to do this on separate tickets in you are MFUing from WT+ to CW, otherwise the onwAArd flight will be booked in whY. If you are MFUing from CW to F then you could do it on one ticket, but I suspect the CW fare to SJO will be around double booking on separate tickets.