Before the keynote, we have no idea what the next iPhone will be like. We can make educated guesses at what the next iPhone might be like but nothing is set in stone until Apple actually goes ahead and announces it.
My best educated guess would be that Apple will:
- Improve the camera in some way (megapixels/sensor)
- Have a feature that is unique to the next iPhone that previous iPhones do not have
- Retain the 4" screen
Ahead of this, any other guesses I might be able to make at what the next iPhone will have would much rather be wild speculations rather than educated guesses.
I've bolded the 4" screen because I've personally experienced screen size as being a major factor in whether people go with iOS or go for a large screened Android phone. You use the screen on a phone virtually everytime you use your phone. If you're finding the 3.5"/4" screen is too cramped for a smartphone screen in 2013 then it is unlikely you're going to like the screen on the next iPhone as it will most likely be a 4" display.
Past that, it all just boils down to personal preference. You seem to have mentioned alot more about things you like in Android in your post as compared to what you like about iOS (simplicity and a conversational digital assistant). It seems to me that you are leaning towards Android and want some justification to push you towards it.
The learning curve on Android isn't that steep (- been there twice) and it is easy to get used to. Yes, iOS is simple and easy to use but that can sometimes become a little stale and boring. Android is more complex but is much deeper than iOS in terms of customisation and user capability.
Don't let Siri hold you back - Google Now is a far better option to the awful software Samsung created known as 'S Voice'. I really only use Siri to reply to messages or make phonecalls while I'm driving - past that, I find it faster to do things on my phone myself rather than to rely on Siri.
If you tend to use a bucketload of apps on your phone, I'm sure you know that using apps on iOS beats Android hands down both in terms of fluidity/optimisation and choice.
One thing to note - When I moved from iOS -> Android, twice, I noticed both times that audio on Android really sucked. I am not an audiophile by any stretch but it is probably my favourite thing on the iPhone when compared with other phones. Apple really put alot of attention into the audio side of things on the iPhone and it is, sadly, something that is often missed in reviews and comparisons. Let me illustrate to you what I mean:
My Desire HD had awful, tinny, QUIET speakers that, for me, were beyond useless. The microphone also sucked and I found it difficult to record lectures whereas previously, sat in the same position, my iPhone 3G performed well.
My Galaxy Nexus had an awful speaker, poor microphone and above all that a terrible DAC meaning that audio through earphones was poor in comparison to my iPhone 4.
My most appreciated feature of the iPhone 5 is probably the audio side of things. The speaker is LOUD, the microphone is amazing in terms of recording ability and it contains a quality DAC that lets me appreciate my music through my earphones.
If listening to music is a big part of your day then be careful which Android phone you pick and have a look at the audio profile of the S4 carefully before buying. Browse XDA forums for information like that because, as I said before, this kind of information isn't covered well in reviews.
If I had to pick between my iPhone 5 and the S4, I would probably pick the S4 for my personal uses. I'm finding the screen on the iPhone 5 to be too cramped these days and the OS is starting to become dull to me. Expandable storage and a removable battery are also features I would find myself using frequently if there were available to me. If you don't mind Android's unoptimised apps and the awful plastic Samsung insists on using on their flagship phones, you will probably enjoy using the S4 - the display really is amazing.