LF 5" woofer and subwoofer VS. LF 6" woofer

[Darksektor]

I need monitors, but i'm in a budget...
Right now i can buy pair of monitors with LF 5" woofer, and later i would take subwoofer to control lows. Or i may save some money and buy pair of monitors with LF 6" woofer.
So I have such question: pair of monitors with LF 5" woofer and subwoofer VS. pair of monitors with LF 6" woofer.
What is the best/optimal way for mixing purpose in my case?

Thanks.
Andy L.