Core power consumption of any Picaxe depends strongly on the clock frequency. So if you don't need a lot of speed it is better to run at e.g. 4 MHz vs. 8 MHz. Even better, underclock them further (on pre-X1/X2 parts this needs peek/poke access to the SFR). The slower you run, the lower the power consumption.
Another technique is to put the Picaxe in sleep mode and only wake it up when it needs to do something.
Apart from that, don't forget that your total power consumption also depends on the load that you are driving. If you want to drive 20mA on a few output pins (total not to exceed ~100mA), then saving half a mA in core power supply current will not make any difference because the total power consumption is pretty much determined by the load current, not the core power.
Compared to all that the difference between 3.3V and 5V parts is probably negligible. I'd rather choose the voltage that works better with any peripheral chips you use (e.g. if they need 5V anyway, using a 3.3V Picaxe will only create unnecessary headache and need additional components - and possibly additional power - for level conversion).
