Between the p324 and the p 424, there is only a $30 price difference... If that's the case, I'd get the latter. Having said that... I'm an aluminum kind of guy. Carbon fiber is nice... it is lighter, but not at twice the cost.
That's the great thing about the Sirui mono pods - they are cheap!
Seriously if you really want an alloy mono pod then get one but I guarantee you will be disappointed in the long run.
I have a Manfrotto 679 alloy mono pod in virtually mint condition, simply because it is never used!
The P424 is a fine piece of kit but really not necessary for a 500 F4 or smaller, that is where the P324 is a better choice - but there is no harm in too much support!