Hey Don! Welcome to NICOClub!
I'll be as concise as possible -
1) Do you want to mod, or do you want power under warranty? I recommend power under warranty, myself. The RS makes gobs of it. It also has a couple of extra hardware components (notably turbo speed sensors and an extra water pump for cooling) which is nice to have should you get into the mod route (I'll touch on that in point 4).
2) The car has garbage tires from the factory. Put some sticky, non-run flats on the car, and you'll find a RWD would be just fine in SoCal. The AWD is extremely nice, however, but given where you live, I don't see getting a RWD as a deal breaker given you swap the tires. What I do recommend is sourcing a RWD with the spare tire package equipped and you'll be golden. Be aware that neither AWD nor RWD can be equipped with a Limited Slip Differential, so depending on your driving style, this may affect your decision. Infiniti uses a type of brake "torque vectoring" (to use the term loosely) called Active Trace Control, which basically uses your brakes to mimic the action of an LSD.
3) I imagine you'll be just fine as long as you stay stock for a while. The aftermarket guys haven't quite come up with a fool-proof solution to tuning and stuff with the Red Sport. There is a lot going on with the car that hasn't been present on previous performance platforms from Nissan, most notably direct injection and electronically monitored turbocharging. There is way more to it that just cranking up the boost. (more in point 4)
4) The aftermarket for this car will be monstrous. Simple bolt-ons are yielding serious power from this platform - it appears Nissan has seriously underrated it. The weak points for power adding are the smaller turbos and the transmission. AMS (of Alpha GT-R fame) are officially partnered with Infiniti, and they will be releasing performance parts - some will allow you to keep your warranty and some will not. They already have an 11-sec Q60 Red Sport that is outfitted with parts soon to be released through the official Infiniti stores. It's still a relatively new platform, so the tried-and-true solutions aren't quite there yet, but they absolutely will be.
As far as bang for the buck goes, Fast Intentions has a catback exhaust for the Red Sport models that is going for around $1300 right now that will get you near 30 lb-ft of torque without a tune. It will be an incredibly cost-effective platform to modify once the kinks are worked out. Watch this space as we'll have some write-ups on some of it.
5) The BMW 340i is an absolutely wonderful vehicle, and quite frankly, the safe choice is you want to get into modifications right away. However, the Infiniti, equipment for equipment, is a more cost-effective way to get into a premium performance-oriented sedan. You'll find the Infiniti less expensive than the BMW. The BMW currently has a stronger aftermarket base, but Infiniti is working hard to catch them. Definitely drive both - both great cars. I believe they're so close that it will come down to which one just feels better to you. However, at the end of the day, this is an Infiniti forum, and you'll get the answer of "Infiniti is a better choice" similar to what you'll see on a BMW forum
Good luck in your search! Let us know how the test drives go!