Are you concerned about "being legal"? because some countries have power limits and you could be fined. If you feel free to do as you please, I would definitely recommend the 350W motor on the rear wheel. Once someone experiences having some electric power, they usually want a little more very soon.
i have 2 folders and would only recommend a front motor if u plan to do 1 or more of the following.
-stay w/in the EU legal power limit - more power on the front fork is not advised, although it has been done
-plan to swap it out for a conventional wheel on occasion - faster/easier to swap a front
-want/need to run original freewheel/cassette (or internal hub) larger than 7spds - most motors can handle a 7spd freewheel in standard dropouts
-rear dropout is to small to accommodate hub motor - then u need to also check front
but thats just my personal preference.
personally i recommend a rear 350w geared hub motor (they're lighter, more efficient & free wheel) w/ a 7spd 11-32 freewheel and a 46-50t chainring at 24volts or 50-53t at anything higher. this way u can still climb hills and keep a modest cadence on the flats.
both my bikes have 20" wheels running 12s (44v nominal) and i can expect spds of 22mph or 35kph - but that will vary a bit from motor to motor.
i have abafang geared motor..rear mount,36v,12 amp ping battery[frame mounted]..motor wt7lbs... bnattery wt,10lbs.... speed on flat 3okph.. infineon 36v,25amp controller and aCA... lightweight and reliable