if it were me, i would buy them. they are available and it gives you a few more tries for a full sibling.
we wanted to use the same donor for all our children since dp and i both planned to carry. however, we didn't bank a lot of vials and managed to luck into 6 more vials before dp ttc. both our sons have the same donor (1 that i carried, 1 that dp carried) but for our 3rd try we only have 2 more vials left. however, we have decided that having a third child outweighs all 3 kids having the same donor. we bought up 12 vials of another donor last year - he has similar characteristics, height, and major as our original donor. it's interesting to note that ds1 looks like me and ds2 looks like dp. we must have strong genes!
having said all of that, in the end, i think if having another child is important to you, it will outweigh having a full sibling. after all, 6 vials is no guarantee of pregnancy!
my family - dp d , ds b (4), ds f (2), dd a (jan '12), ddog m