just a suggestion, but i'd recommend a wet/dry sump over canister filters for rays. much less cleaning work and much more biofiltration.
i hope you are able to find a hystrix. they are one of the harder to find types out there, especially due to brazil's exporting restrictions. i've been looking for ages for sources for them (where i don't have to pay an arm and leg for shipping at least) but no luck. online store would def be your best bet.
hystrix would do well in a 300gal.
as for male vs female, hard to say. my male ray is skittish to me and has a temper towards everything else. my female is completely relaxed and has a great personality. he looks nicer but she has the better disposition, hands down.
my lfs had a female motoro that within a day of being added to their tank, killed another ray and was just down right nasty.
just like people, each ray will be different from the next. i would say go with whichever looks the nicest if you have the choice of which to buy. if you plan on doing adding anymore rays down the line and trying for breeding, i'd suggest getting the female first, you can grow her out, and by the time you add more rays, she can be nearing or fully matured, of the right size, and ready to breed. having a mature female in the tank also seems to speed up the sexual maturity in growing male rays, so to start the breeding quicker.
just my 2 cents
