depends on your budget and what you are trying to receive
lo stability -+250 khz is a very good reading
most higher end lnbs have this ( norsat ect)
they increase in readings of 250 and the price drops
so if you have the money and want your channels to lock lower ratings are best
for me i just bought the bsc621 and i think its rated at -+1 Mhz but its a inexpensive lnbf for me to do some testing with
my main point for posting was just to let you know to check out the specs of each lnb cause a 13K lnb might not be as good as a 15 or 17K lnb if the L.O. Stability is high