If you change the collateral rate, that would not increase PnL. If market moves against the bank, no collateral will be posted and the increased collateral rate will be irrelavante (assuming one way CSA), and if the market moves in favour of the bank, the client will post collateral that will be payed by the higher rate.
If you change the fixed rate, that would directly reflect in an increase of PnL, roughly 1bp times the PV01, so I would go with the higher client rate.
Update: Considering that at the moment you trade the derivative should be close to fair value, I would assume the initial MtM is close to zero for a zero fixed rate. After that it depends on the direction of the market, and even then, both rates would affect your PnL differently.
Changing the swap rate would affect the PnL directly (1 x PV01) but changing the collateral rate would have a much lower impact.
Consider a notional of 10Mln on a 5y swap, the PV01 would be roughly 5k, so if you increase the swap rate by 1bp the PnL will be 5k. If market moved 1bp the posted collateral would be 5k and the daily marginal return on the collateral after changing the collateral rate would be 5000 x 0.0001 / 360 = 0.001389