It comes down to budget,
Brass props are very shiny and nice on display stands, and they function just as well on a working model, a lot of brass props are created for models where the plastic range cant or don't cater for, Kortz props, sub props etc..
Plastic props are fine and a much cheaper option to the brass counterpart, as an example, a 30mm Brass 3 blade prop would cost you £10 each where as the plastic equivalent will cost you around £2
Don't think just because they are plastic they are flimsy and weak you'll be surprised how tough these props are.. Also a good thing to remember is if the boat throws a prop (which is known to happen) I'ld rather loose £2 in the lake then £10 any day of the week, that being said .. Do the Nut up Tight To The Prop!!!
Prop size however for you boat needs to be considered, what motors are you running in your boat?
As an example a 400 sized motor will happily turn a 30-35mm prop at 6v but the motor will run ''warm'' suggested no more then 30mm for a motor that size, if using a motor 555-600 or 700 then anywhere up to a X45 size prop will be fine, any motors from the 800+ (including the T12 & T24 from Mobile Marine) range any where from the X40, x45 up to the larger 50, 70 & 80mm props will run.
Reason i mention prop size is to do with Amp Draw on the motors which in turn making sure you have the right ESC to do the job.. if you over load a motor with to large a prop, when the boat runs under load ( so put in water) you can blow the ESC if you are drawing more amps because you setup the electrics wrong.
There is No load Draw and under Load Draw its always Under Load Draw you need to pay attention to.
For 400 size motors and under ( were talking brushed motors :DC, Not Brushless) then a 10 Amp ESC will be perfect for the job up to 12v
555-600 size motors at least a 15 amp (20 amp is prefrable and price is Pennys diffrence!!) esc to turn the larger props at a good RPM.
T12 motors and 800 up to 900 sizes you need at least 30-40amp esc's to be on the safe side.
Subject to correction, i would explain Brushless motors and ESC's too at this point, but until you let us know what your planning to power you boat with I'll keep to the 'basics'
