Two big issues affect the choice of frequency (propagation issues aside).
The first is whether you will be interfering with someone else's transmission. This includes military operations, utility stations, hams, and other (possibly) licensed entities. It could also mean other pirates; sometimes there are pileups on 6925, when no station can be heard easily. Interfering with a non-pirate station, particularly the military, is a very good way to get busted.
The other issue is whether anyone will be listening. This used to be a lot more important than it is now; it once was that picking a random frequency that was not widely used by pirates meant that you stood a good chance of not being heard. Nowadays, though, someone with an SDR will likely notice that you are on the air, and the news will be spread via web sites like HFU, as well as chat rooms such as the Starchat #pirateradio channel.
6930 is likely a good place to be heard, since a lot of people monitoring 6925 will scan through there if they are using a proper radio with knobs, or see it on their waterfall on their improper SDR. Just be sure to listen a bit to make sure you are not going to step on someone else.
6925 is like a box of chocolates....
Would 6930 be just as diverse as 6925? And would a person broadcasting on 6930 have as much chance of being heard as if they used 6925?