I had a discussion with OM Darrel AA7FV on why many USA stations are so strong and hearing me better on 40m than 30m, when this should not be the case. If you see my antenna at vk5eee it is clear that 30m should perform better on this antenna. According to propagation charts there is also no clear explanation. But the explanation is quite simple:
On 40m, I'm hearing, and those that are hearing me, are the big contest guns. Ellen W1YL/7 for example is using a very high powered contest station via remote. There are many high powered 1kW+ USA stations with multi element beams on 40m. On 30m it is different. There is less activity and the power limit is 200W. Those with beams on 30m are much fewer, and those beams are not optimised for contests e.g. at exceptional heights.
So, if we compare those using modest means, then we can get a real comparison, and there, there is little or no difference, other than 30m is a bit easier to achieve a decent signal given the smaller wavelength, and easier to achieve height. Also down here in Oceania, 30m is often very quiet with little activity to be heard, when compared to Europe and North America, where you can hear many stations with single or two hop propagation from modest antennas.