If you run both cabs at once, you shouldn't have to worry with either the 350 or 500. With one 8Ω cab hooked up to the P350, it will deliver up to 250-watts rms. If you hook up two 8Ω cabs that will put a 4Ω load on the amp and you'll get all 350-watts out of it. That 350-watts is NOT 350-watts each but 350-watts total split 50/50 between your two cabs, which is 175-watts into each. So using even the Laney cab by itself you should be fine because it won't be more than its 250-watts rated handling power.
If price isn't too big of a difference the 500 will give you more options. It puts out 300-watts into 8Ω or 500-watts into 4Ω. You could safely run the Warwick with that 300-watts into 8 ohms but it is "possible" to blow your Laney cab if you turn the amp all the way up to get all 300-watts out of it. If you run both cabs from the PF500, then the cabs will each get 250-watts and you'll be fine. Personally I'd get the 500-watt version and just be a little careful about not maxing out the master if I'm only running the Laney Cab. Actually, what I would suggest doing is just running your Warwick cab whenever you are going to just use one cab and add the Laney on whenever the Warwick cab by itself is not enough and you need to run two.
I personally ran a couple of Eden 112 cabs that handled 300-watts rms each with an amp capable of putting 400-watts into each of them. I used those for my medium sized venues for about a year with no problem. Now, I run a really clean and clear sound and I don't boost the bass. Nor do I run any distortion. I boost the low-mids up to about 1:30 and otherwise I run them flat - got complimented on that sound a lot during that year and never came close to maxing out those speakers. I think 10 o'clock was the highest I ever ran the master on them. I used my DNS-410 cab on the large venues.
Reason I mention the bass EQ is that if you boost that a lot, you can blow a speaker without ever coming close to its maximum power-handling. That shouldn't happen but real-world, it can and does. That can make the speaker exceed its XMax rating (farthest it can move without distorting) and in a worst case situation exceed its XLim, which is the extreme limit the speaker can move without damage. Go past that and things crease first and then they tear. So don't be too heavy handed with your bass EQ.
If you ever hear your cabs make farting noises, immediately drop the bass EQ and if that doesn't clear it up drop your master until it goes away. And when you hear that, you have to make the change NOW! If you wait until the end of the song to make your changes, you will have already blown the speaker(s). If you are using distortion, it's all but impossible to hear that before the speaker is ruined. If you run a clean signal, you at least have a chance to save it.
