I don't think a straight answer to this Q is possible, because the mission profile changes (towards greater risk) once a more capable aircraft and additional training (and bits of paper) are acquired.
If you set off some plain-PPL pilots on a perfect-day VFR trip down the road, and similarly with a bunch of competent instrument pilots, I would jolly well hope the latter would be safer.
But that's not a useful comparison.
A better one would be to send the two groups from say Lydd to Trondheim (Norway). The VFR ones go VFR. The IFR ones go any way they want.
In that case, I would say the instrument group would have a much better survival rate, not because they are really smart but because the basic VFR-only training is really crap for any real-life long range VFR trips.