Don I hope I explain this clearly ???
Theres a clear relationship between , volts - Ohms- amps , Ohms Law calculates these relationships ( V over I/R ) where v = volts , I = amps of current , and R= resitance in Ohms , all of electronics and electrical is based on this.
Power /watts is calculated based on this relationship.
Example : if the V is fixed @ 6 V ( like a battery ) and the R is 3.5 ohms the I =1.71429 A , and the power is 10.28 watts. play with the calculator link it will help you to understand the relationship.
More power = more heat from a heater coil
The attys I have checked are around 3.5 ohms 901,510 ( only ones I have tested ) I think Nerf has a list of atty OHMS ?
Attys have ohms
batteries are not looked at that way ( they have an inherent resistance )
Batteries are storage mediums rated in Volts and Amp hours ( or mah , 1000th of an AMP HOUR ) your 250mah batt has more storage capacity than you 150mah , a 250mah batt. can run 250ma for 1 hour until it hits a rated discharge amount ( I cant remember but I think it 60% or so ???? dont quote me on that ! )
I hope this helps and does'nt just cause MORE comfusion

LOL ! , if you have any other questions just ask me I have many years in the electronics area.
There are some WAY OUT THERE statements made around here regarding V/I/R !! and some seem to think that there is MAGIC involved , but it is as simple as OHMS LAW ! ALWAYS !!!!!! ( HELL your cell phone would'nt work without it )