iOS has full W^X (aka DEP), Android does not. This prevents JITs on iOS, but it also makes exploits more complex. iOS also has full ASLR (even kernel-level ASLR likely coming in 6.0), Android doesn't to the same extent. iOS also has a more restrictive an low-level sandbox, which denies access to many kernel interfaces; Android has a sandbox, but less restrictive. iOS also has app review; with Android, malware could (and has) easily enter the user-facing market. All code that runs on iOS must be signed by Apple, this significantly hurts exploit development as all code must be written using "gadgets" from existing, signed code. Android will run anything.
Android also has the issue of updates: most Android 2.x phones are not going to ever get the improvements in later versions.
Can't comment on whether it's more or less secure than iOS. What I can say is that it's openness is not only it's biggest strength, but also weakness.
Quite simply because a network operator or handset manufacturer (often both) make their own changes to what is, I imagine, a relatively secure base platform.
That requires users to trust the NetOp and manufacturer's intentions and, assuming that's all good, users must trust that the NetOp and manufacturer's code is secure.
My trust, like my respect, is earnt. It's not handed away on a silver platter.
> Android on mobile shows us how spyware and viruses are a very big problem without those restrictions, compared to, say iOS
Which I took as saying Android is inherently less secure, but I agree after your clarification that isn't what you were actually saying.
That said, this doesn't show Android is more susceptible to viruses than iOS devices, and iPhones are not immune to botnets either (http://nakedsecurity.sophos.com/2010/03/09/8000-iphone-andro... - though this only affected jailbroken iPhones). Previous jailbreak exploits that worked through Safari could have been disastrous as well - it's not that iOS is more secure, but I do agree it seems to be less targeted. As the popularity of iOS seems to be on the rise I believe this will change.
