Great question, I usually cook for 8 to 10, with seasonal spikes(holidays, Super Bowl, etc). Realistically trying to stay under 600
Thanks! That helps! I hate giving recommendations without the parameters :-D
For $600 you have a few good options, and you can afford a combo of two if you want...
In no particular order...
22.5" WSM - give you plenty of capacity for you usual cooks and room to grow for those spikes. Add a controller if you want for peace of mind.
UDS - Same advantages as the WSM. You can go completely homebrew or use the Big Poppa kit as a starter.
PBC - Easy to use and can cook for 8-10, but it would be tight. May not handle the spikes as well, especially if you are cooking something that doesn't hand well.
26" Weber Kettle - Great for grilling and can be used for smoking with good results, but not as easy to use as the WSM or UDS, and the capacity is not as great.
Big Green Egg (and other Kamado type cookers) - The most versatile. Can grill at lava temps, bake and smoke. You would need a Large size at the least, so buying a new BGE is well over the $600 budget, but if you have patience you can find one on Craig's List. You could also look at one of the lower cost Kamado cookers that have come out recently. Cooking for 8-10 would be a challenge, however. You would need a second level grate at least.
So... My recommendation?
A 22.5" WSM and a 22.5" Kettle. That give you the best of both worlds and is under your budget, especially if you can find either on Craig's List.