I have always gone for the top performer in any car I've purchased, and always love V8s. However the 335is is more than enough performance for most folks and was the more afordable choice over an M3, purchase price and fuel cost.
However, the cost isn't much different in the overal equation. For example, at 1200 miles per month, 20MPG = $240 and 24MPG = $200, a little more than $1 per day. A small price for the pleasure of the V8 sound and performance. OTOH, a 15MPG M5 or big SUV would be $320, noticeably higher. In the other direction, the savings aren't that great. 30MPG = $160 and it takes 40MPG to get down to $120 per mo. Note this all assumes $4 per US gallon where the fuel price can make a bigger difference in the monthly cost than adding a few MPG.
2013 X3 35i, Vermillion Red/Sand Beige: MSport, Premium, Technology, Premium Sound, Hud, More...
2012 335is, Space Gray/Oyster-Black: DCT, 19s, Premium, Convenience, Nav, Heated Seats, More...
Gone '11 X5 50i, '09 Z4 35i, '08 550i, 06 X3, 06 650i, '02 M5, '99 540i