It's my wife's primary vehicle and I primarily drive an Acura RL. I think the 28i has plenty of power and if it had more I'd get speeding tickets. I like the fact it gets better gas mileage and costs at least $3K less up front. If you're an enthusiast go for the bigger engine, you're wife will doubtfully see a difference. Just got back from a long drive over the weekend and we were getting 25+ mpg. At times the on board computer was registering 30+ mpg. Bottom line is we're filling up the tank about half the time over our old Acura MDX. Insurance rates are less with smaller engine.
I think it's always a good idea to do a back to back comparison test drive of new model vehicles.
2014 528ix M Sport. Space Gray/Mocha Brown Nappa Leather, Anthracite Wood Trim. Packages: ACC Stop & Go + Active Driving Assistant, Driver Assistance, Driver Assistance Plus, Lighting, Luxury & Premium. Options: Paddle Shifters, H-K. Surround Sound System, Heated FS's & Power Rear Shade with Manual SS's.
2013 X3 28i M Sport. Deep Sea Blue/Sand Beige Nevada Leather, FS Wood Trim, BMW Apps, HG Rails and GY LS All Seasons. Packages: Cold, Conv, DHP, Perform, Prem, Prem Sound and Tech.