A foreword: I don't have a NEX 5 or NEX 3, so take this with a grain of salt. I might be wrong about something, just let me know.
Technically, either would work fine if you want to learn about photography. Sony's NEX 3 and NEX 5 have manual control over shutter speed, aperture, and ISO. To quote from DPReview's review (which I would highly recommend reading - DPReview is very thorough),
Sony has made clear that it is aiming for compact camera users who wish to upgrade (a market it estimates at around 10 million potential buyers), rather than trying to offer a second camera for existing DSLR users. And the NEX models have more in common with compact cameras than DSLRs - including very few buttons and a resolutely unconventional interface.
It looks like you can get access to the most important functions — shutter speed, aperture, and ISO — through the dial and maybe a button press or two, which is good. To me, the dial looks awkward compared to an SLR's wheel(s). However, the less-frequently-adjusted-but-still-important options like AF mode, white balance, metering mode, shooting mode (P,A,S,M), etc. seem to be buried in menus. Once you start to learn the technical aspects of photography
- There only seem to be 3 lenses available now for the NEX series. Since these are all still in production, and have only been out for a few years, it will be difficult to find used lenses. You can get adapters for several mounts, including one that supports autofocus, auto-exposure, and auto flash for the Alpha system, but that's one more thing you have to buy. DSLRs, especially Nikon, Pentax, and Sony, have a wide range of used lenses which can be much cheaper than the new models.
- No upgrade path. If you want to move up from your consumer-oriented camera body into something with a few more buttons, dials, and features, you'll have to switch to a DSLR line and buy a whole new set of lenses.
- Doesn't have a standard hotshoe. It does have a "Smart Accessory Terminal" to which you can attach Sony's small external flash. You might be able to find an adapter, but there doesn't seem to be one yet.
- Contrast detection autofocus. This is the autofocus system that most P+S cameras use, as opposed to the phase detection autofocus system used by most SLRs. Generally, contrast detection AF is slower than phase detection.
- With a zoom lens attached, too big to put in your pocket, but still smaller than a DSLR.
- Fairly pocketable with the 16mm f/2.8 lens attached. This is somewhat fast and very wide angle. This might be appropriate for parties and other dark, crowded social gatherings. With a DSLR, there's always the risk that you won't have your camera with you when you want to take pictures because it's big and heavy; this might get around that, to some degree.
- People won't be intimidated when you ask them to take a picture for you — it looks like it handles mostly like a point and shoot.
- They actually seem to have decent build quality — the lenses are metal-cased, and the NEX-5 has a magnesium frame, something you don't usually see on sub-$1k SLRs.
If I were just starting out in photography now, I'd probably go with an SLR or a hobbyist's compact like the P7000, G12, or LX5. To me, the NEX series has the inflexibility of a compact camera, combined with the lack of portability of DSLRs. That's just my opinion, though.