I would much rather have TWO matched 150 watt-second monolights than one, 300 watt-second monolight and a speedlight. 150 to 180 watt-seconds when used with a 7 inch reflector and an umbrella or softbox is ample for using with today's d-slr's, which have excellent image quality at ISO settings from 200 to 400.
I think you'll find that a 150 to 180 watt-second monolight is more-powerful than your SB 600, which I would consider to be around 65 watt-seconds roughly, compared to a monolight using a 7 inch reflector. So, take heart, a 150 to 180 watt-second studio monolight is more powerful than you might expect; I have a lot of studio lighting gear that uses the pack-and-heads style of a central power pack and independent flash heads, and I often divide 400 watt-seconds through 4 light heads,and that is enough for many portrait uses. I have a JTL 300 watt-second monolight, and I almost NEVER use it at full power--it delivers too much output for my uses. I really would encourage you to buy two identical, somewhat lower-powered flashes rather than one 300 and one speedlight.
I think it is really nice if the reflectors on the monolight can be fitted with a honeycomb grid attachment and or barn doors. Not essential, but a very nice feature.
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