Steam has two major advantages:
Steam Sales
(edit: this is less true these days, other online stores now have huge discounts regularly too) 75% - 90% off most major titles within 6 - 12 months of release. There is nothing close to the deals steam have, not on online shops, not in bricks-and-mortar shops, not even second-hand.
Valve's reputation
Valve have proven time and time again over the years that they are "the good guys". They make a lot of effort to do right by their customers. So I trust them (more than any of the companies you listed) to not lock me out of my games if they can help it. If Valve ever got sold and decided to cancel Steam forever, they're the company most likely to secretly release a hack or something to keep my games running.
They are also extremely profitable, thanks to their excellent games (I mean the first party titles like HL, L4D and Portal), and to their excellent business model. They are the digital distribution company most likely to stay in business forever.
I found Sega Genesis / Mega Drive 101: A beginners guide on Racketboy a great resource.
Is the Genesis exactly the same as the Mega Drive?
The name is just a trademark issue, it was originally the Mega Drive, but was marketed as Genesis in North America since the name had already been registered.
What's the difference between the 1, 2, and 3 versions?
Model 1 - Only version to include a volume control slide switch for the stereo sound output. Supposedly, this model is the easiest to perform modifications on for regional bypasses, overclocking, LED change and S-Video output.
Model 2 - Smaller, simplified design. Stereo output is included in the A/V outputs and there is no volume control for the sound.
Model 3 - Even smaller, harder to hack, not compatible with Sega CD or 32X.
There's a forum thread discussing which of the three is the best model. There's even more information on the different Mega Drive systems on Wikipedia:
During its lifespan, the Sega Mega Drive quite possibly received more officially licensed variations than any other console. While only one major design revision of the console was created during its lifespan, each region has its own peculiarities and unique items, while other variations were exercises in reducing costs (such as the removal of the little-used 9-pin EXT. port) or expanding the capabilities of the Mega Drive.
Variations of the Sega Mega Drive
The article covers all of these variations.
Can I buy a Genesis, and then buy cartridges either for the Mega Drive or the Genesis?
The games are locked per region, but there's a good point on this:
System modifications such as adding region switches and SCART outputs can be readily accomplished on most model 1 and model 2 hardware.
Coincidentally I was considering buying a Model 2, which is also the one with the most votes on the mentioned forum thread, so I guess this pretty much answers my question.
Best Answer
Both the Genesis and Mega Drive collections are NTSC versions, widely regarded to be the better ones due to the fact that PAL versions usually ran slower.
However, there are a few odd things that seem to be running at PAL speed (50hz rather than 60) like the announcer's voice in Virtua Fighter 2. This is probably just an emulation issue.