I would choose one that has the yield and transaction costs you like. Better to buy 100 shares of a $100 stock than 1000 shares of a $10 stock as far as the commissions are concerned.
Also, I don't think you can buy t-bills at the auction through IB unless you're doing 1 MM or more.
The only problem with the long stock and short SSF strategy at IB is they don't let you do it in an IRA account . This doesn't make sense since the let you write covered call in an IRA.
There are many ways of doing that in TWS, the simplest one is by using the SpreadTrader , type in the underlying and choose "EFP" flavor of it.
If you don't have the account with us you can use our demo.