It's not a lot - but from my approx. 550 in the 40 and 150 in the 45 here are my thoughts:
1. My understanding is the 45 is $3M more than the 40. Their is no APU, 2 less seats, and 800 lbs. less fuel in the 40. This does create a W&B issue with only 2 pilots on board and full fuel (have to put your bags in the front cabin with only 2 pilots - not a big deal)
2. Both the 40 and 45 have fairly good avionics (Honeywell) and are user friendly. The standard Universal FMS in both works, though I'd prefer to use a Honeywell FMS with a Honeywell system. No APU means you'll be on ground power everywhere you go in the 40, which I never found to be a bid deal at all. It takes a lot of ground power fees to pay for an APU.
3. In all reality, the 45 only has about 30 more minutes of cruise time over the 40. I've found that I need to "start down" in the 40 at 3 hours. In the 45, I need to "start down" at 3.5 hours.
4. The 40 (non-XR) does well in both takeoff and landing. In other words, takeoff distances and performance were acceptable for most IFR departures with fuel, bags, and people on board. Once above about 3000 the bird starts to lose it's 2nd segment climb numbers - then again, what airplane doesn't except the really expensive ones. On landing, the TRs work great and the brakes are KILLER. You can get the thing into fields you can't get out of!
4. Maintenance: our 40 was on warranty. I personally wouldn't buy either a 40 or 45 unless it was under some warranty. We had an engine oil issue (swap and fix), flap issues (had to replace the motor finally), a door-strut problem (think a factory repair came out for this), a screwed up throttle quadrant that stuck us in NOWHERE, GA once (sent factory rep out to fix it), nav/com issues (red warnings, not sure what was exactly wrong), and many interior issues (soot on the ceiling, airshow constantly not working, armrest breaking, etc). Our 40 had 1300 hours when I started flying it and about 2300 when I stopped flying it (it was on charter). The 45 was not much better on maintenance - especially the APU which never worked. It, too, was very low time and at Bombardier often for repairs.
5. Ours was insured for $7M. Personally, if I could pick one up for $7M to $8M on warranty, I'd go for it. When it works, it's a great bird you can pack with fuel, people, and bags and go somewhere. It won't scare you and the passengers will love it. Feel free to PM me if you have any more questions - most of this was off the top of my head.
AZT