+1 on above regarding the merits of each block.
IMO, the Romeo is better for the simple reason there are more of them available configured in "car" trim.
In terms of numbers, Windsor engine in a Mustang was a two year thing (1999-2000). Which can make it difficult to find direct replacement engines/parts. Where as the Romeo engines are common in Mustangs, Grand Marquis, Crown Vics, Town Car, Cougar, and Thunderbird.
The Windsor block is available in truck applications. However, for a salvage motor source, transplanting a truck motor into a Mustang (while doable) does bring extra "problems" to the table (intake, timing cover, flywheel, belts, yada, yada). Problems that are not present in a motor preped for a car application.
The Romeo does have a removable cam gear. This makes degreeing possible.