Double-edged sword, isn’t it.
In the Android camp, you don’t get (relatively) many model-specific accessories, so you often have to make do with “universal large smartphone” mounts etc. However, when you upgrade handests the chances are that everything you own will still work / fit.
With Apple, because there’s only one device to cater for, things like Graham’s bike mount, radio docks etc are exactly tailored for that device. However, when the iPhone 5 (or whatever) is released, everything you own which interacts physically with the device will need to be either adapted or replaced. Unless, of course, you bought generic accessories in the first place.
Historically the Apple devices have been (largely) electrically compatible, so typically a docking station would be supplied with a selection of plastic surrounds to secure the 3GS, 4, 4S etc. But there’s no guarantee that this will always be the case, as the rumoured new port would attest (and no guarantee that third parties can or will release new adapters for old add-ons, either).
Which is more important is really a matter of personal preference. Personally, I think it’s a non-issue either way. I rarely if ever plug my phone into anything other than a power source and I can live without a wide selection of Hello Kitty phone cases (TBH I don’t really understand why people would pay a premium for a sleek, slim device and then immediately double its size with some cheap rubberised cover and an ill-fitted bubbly screen protector, but maybe that’s just me). So in terms of influencing my purchase choice I’d choose an iPhone, Android or a.n.other device based on their relative merits, the availability of after-market accessories and assorted tat is fairly low on my list of priorities.
My OH has several cases for her iPhone, I have the official HTC slip case for my Sensation. For her, choice is important, in the same way that many girls desire n+1 pairs of shoes. For me, I just need a case to protect my phone, so long as it looks vaguely professional and doesn’t add too much bulk, I’m sorted; it doesn’t need to match my handbag. Horses for courses.