I think if you have the chance, you should always promote, especially if you are talking D5 to D4 USA. In D4 leagues, there are often a good number of new teams that can end up going bot, or "lightly" managed teams. Their presence should allow an actively managed team to stick around and avoid relegation. This will give you the chance for more revenue, but still allow you to train players intensively.
My own team spent two full seasons in D5, which I don't necessarily regret, so that extra year can be put to good use. Plus it did allow me to get through D4 in one season.