+1 to everything markymarkspider said
In terms of sound, in my experience the 12 Valve V6 sounds a little 'angrier' than the 24V and also sounds a bit smoother, wheareas the 24V sounds a little bit deeper and perhaps more civilised but still sounds good. It's quite difficult to describe the difference in noise really, you'd have to hear it for yourself, or try Youtube! The two engines perform differently too - comparing a 164 12V QV to a 164 24V QV, the 12V is smoother than the 24V and has great low-down torque, but the 24V is undeniably more powerful, revs more freely and is overall a bit more of an animal. The 24V engine also responds to tuning better than the 12V, which is limited by it's cylinder heads and highly restrictive cast iron exhaust manifold (though I think your spider might have better stainless steel manifolds as standard? Possibly made by Zeuna, who made the 164 24V factory manifolds which flow very well). As you might expect, the 24V heads are a better design for getting air in and out.
Some fast road cams, a decent free-flowing exhaust and a remap might generate between 10 - 25 ish hp on an 12v V6 engine that's in good condition - this is very difficult to estimate though, there are so many contributing factors to consider. I would be surprised if the increase was more than that. As markymarkspider says, one of the best ways to achieve a reliable
250 hp is via supercharging; another way would be to overbore the engine to 3.5 litres. This is, of course, expensive! But a 3.5 12V with a modified cylinder head, decent cams and exhaust, and a remap (or standalone engine management) should provide over 250 hp without too much trouble. A 3.5 24V with the same modifications will be nearer to 300hp. My 164 Q4 (3.5 24V) supposedly produces 280 hp and 342nm of torque, and this is with standard cams, standard exhaust (including twin catalysers), standard air filter, standard intake runners, and so on.
I will end this post by pointing out that all of this is, of course, hypothetical; there's no guarantee of specific gains to be had from specific modifications. The only way to know for sure is to take the car to a rolling road/dyno to see what the current hp is before you modify it, then return to the dyno after each modification or group of modifications to see what the gains are.
I hope that helps.