Sr. Open division is tricky
-It can have up to 36 members and moste competitions we wnet to they didnt split the division into large and small
-It can be coed with up to 4 males
-division can vary but there are almost always at least 3 teams in this division.
-doesn't have an age minimum so teams can put up little bitty 6 years olds on 18 year olds

If you arent needing to use the Open division because of age (having a 9 year old who has the skills to be on the senior team) then I would suggest going into the regular coed division. You run the likelyhood of competing against other coed teams anyway so why not. I think the biggest point is, that being in the Open division you will likely be competiting against teams of 36 without the division splitting.