The CX-5 diesel hasn't come out yet, so there is no opinion on the powertrain. It'll probably be a pretty good diesel because Mazda knows how to make a fun and efficient powertrain.
It all depends on your taste. The Forester has more off-road credibility with standard AWD, road noise, a roomier interior, a CVT, probably a more upright driving position, and a rugged, durable interior. The Mazda CX-5 has a much sportier driving experience, a smaller but much nicer interior, great handling, good looks inside and out, less road noise, and an available AWD system.
I much prefer the CX-5 personally because I've never been a fan of Subaru (boring IMO), it doesn't snow where I live, and the CX-5 is just plain better in almost every way IMO. There are still people that won't think it's for them. If you want a no-frills point A to B CUV then the Forester is OK. Otherwise go with the Mazda. They are both reliable CUVs (the Mazda being more reliable), but the Forester is more practical in some ways. Subaru has better visibility and many say a better driving position, while the Mazda's interior surrounds you and you may feel a littleclaustrophobic.
The diesel should be more torquey and you won't have to rev the engine a lot like in most Mazdas. No one knows about reliability and the first year of any car or engine is always a gamble because they are working out the bugs. I don't think the diesel would be under $25k either.
I'd consider the CR-V also, which I like as much or more than the CX-5. Very comfortable and quiet with high MPGs.