My current bass rig is made up of a Peavey Tour 450 amp and a Mesa PowerHouse 6x10 cabinet. Last night at practice I was barely at quarter volume and both guitar players and even the drummer asked me to turn it down a bit. They are both playing 4x12 cabinets powered by Blackheart and Orange amps and the drummer who has a pro kit. I turned down because I am not a jerk, however inside I was smiling. I love my Mesa cab and my Peavey 450 is only feeding my cab half of it's max wattage potential. Just like if I had like a 750-900 watt amp how loud it would be.
In your situation if competing with loud guitarists, I would say a 8x10 is a sure thing to get at that point. Paired with an amp with enough wattage to peak it's potential. If you can afford it that is of course and always try stuff out first if possible.