I've had both on my car & now run with an Alta, the reason I changed from the Dave F was only because the Alta enhances the supercharger whine more & I managed to pick up one for sale on here for a bargain price

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The Dave F will give you a guaranteed few more bhp & is a very well put together CAI,
Last dyno run I did was at Lohen in October, my car ran a best ever 228bhp, mods on my pre facelift R53 are:
Hartge re-map/15% pulley/Alta CAI/Milltek manifold/Hayward & Scott exhaust/GRS tmic
If you are worried about mpg then a tuned R53 probably isn't the car for you