I don't recall you saying if your 40" TV is 1080p or not.
If so, get both the ATV2 and the BDP. That's what I have. Several reasons to go for standalone Bluray player, some of which will directly apply to you right now, and some which will apply should you choose to upgrade your sound system:
1. Picture quality of a well-mastered Blu-ray is leaps and bounds above anything else in HD. By the way, the scale of pq, from best to worst, is: Blu-ray, Over-the-air HD (via antenna), cable or Satellite HD, streaming HD, Over-the-air SD, cable or Satellite SD, streaming SD.
2. If you have a 1080p HDTV, Blu-ray is the only way to really see that detail and color depth.
3. Sound quality for most Blu-rays is encoded with lossless soundtracks, either Dolby TrueHD, or DTS HD Master Audio. These are bit-for-bit identical to the studio masters from whence they came. If you invest in a good quality A/V receiver and speakers, you can create a jaw-dropping theater experience at home.
4. Most new Blu-ray players stream content, just as the AppleTV does. So, you'll get Netflix, Youtube, sometimes CinemaNow, Pandora, even Facebook, Amazon's video service, Vudu, etc.
5. The prices of Blu-ray players and titles has dropped into bargain basement territory, under $99 for players, and under $20 for a LOT of good titles. Try high def digest online for a ton of blu-ray reviews. That way you can avoid the detritus while investing your money well.
So, it all depends on what is important to you, what hardware you have and what hardware you want. But, keep in mind that, like Apple products, home theater products are VERY Aspirational and can quickly drain your wallet if you let it.