After a quick look at the vacant teams available when purchasing new teams, there doesn't seem to be more vacant teams in RT leagues than in TRAD.

The maximum # of vacant teams in any RT league is 5 (only two of those) and 5 out of 24 is 20%.

There are several TRAD leagues with 4 vacant teams, and 4 out of 20 is also 20%.

The average is between 1-3 vacant teams in TRAD leagues (most of which are 20-team leagues) and the average is 1-4 vacant teams in RT leagues (all of which are 24-team leagues.) More teams in total equals more teams vacant per league.

If someone wants to do a detalied analysis, I'd be interested to see what the averages are, but there doesn't seem to be a huge difference either way.