The OP also stated that she is a beginner (green runs) not a intermediate (blue runs) so 3x as many blue runs on the upper Whistler mountain may not do her any good.
Town of Banff vs town of Whistler. As another poster mentioned, Banff is a real town, although downtown is, as it always has been, kinda touristy. Whistler otoh with 62k beds is not a town, it is a city with a skyline and has such fantastic shopping that you could call it a shopping mall that you can ski at.
No worries about the rain in downtown Whistler or on the slopes mate as they sell green garbage bags with their WB logo on them and arm and neck holes already cut.
A beginner who was listening to advanced skiers, likely why Kicking Horse and Lake Louise got mentioned to her as advanced skier destinations. And typically reason folks go on ski trips is to advance from beginner to intermediate to expert.
Current Banff is about as "real" as current Whistler, both old towns rebulit for ski resorts. Whistler did provide some things the original poster did ask about, lots of equipment rental choices and no need for long waits and long rides on shuttle buses.
Since SNOW is the main feature of Whistler weather in March per the stats vs. people's recovered memories, I wouldn't worry too much about it. Typically hop in the Gondola and go above it on the off chance it's rain vs. snow. Same issues in Lake Louise, just less area to ski above the rain and then there's the two hours of waiting for buses in the rain, driving two hours in the rain every day...ugh.
The snow gods have spoken, colder, more snow...typical.