3" flexwatt comes in 6 and 10 watt,i can only recommend the 6 watt,it will produce all the heat you need.the 10 watt can get up to 150 degrees and is potentially dangerous.
4" flexwatt is 8 watts,most likely not needed.it would help if you mentioned your use,room temps etc.
a simple search "flexwatt" will yield many results.