The mini uses a 14500 battery and the biggest mAh I know of is 750 (which is what I use in mine). This amount of power can get me through almost a full day....sometimes less if I chain vape. Very rarely do I go through more than 3ml a day but I have done that. I think you can safely say you will go through 2 batteries for every 3ml of juice.
The grand isn't much bigger, just like you said - but the battery (18650) has more than enough mAh's To get you through a full day easy, even if you chain vape. One thing I find is that the grand also hits harder than the mini but that is all due to the battery power.
On the reo forum I've seen it swing both ways for either mod. Someone with a grand ends up wanting something smaller and the mini owner wants something with longer life. Its a tough one to decide on. I got a mini first and bought a grand a month later. Then as a birthday present to myself I bought another mini

. I use each one regularly and have them all filled with my favorite flavors. It doesn't get much better....for me
*edit - where in the lower mainland are you? Maybe I can meet you somewhere half way and I can show you what a mini and grand look and feel like in person to help you decide
*edit again!?!? i just re-read your location and I see you're on the island
