Personally I cant imagine not using swype which is free and hugely faster for text input. I have the pay version of swiftkey as well and use that for more technical typing because its a little faster there than swype. Either way its faster than either of the stock keyboards for Samsung or HTC. Same goes for the browser, neither stock browser is one I would use compared to Opera. I can go on and on in the same vein. Doesn't matter which one I would get in terms of stock software, I wouldn't be using much of either.
Not sure on S-voice, I don't use it and in fact already have it frozen with Ti. Didn't use siri either once the novelty wore off.
The performance of the phone is troubling though, it clearly stutters in the launcher and at odd times most everywhere. The S3 did this as well as already pointed out and IMO this is likely to get fixed in firmware soon but in the S3s case it was much less. My impression from reading what people have gleaned from the kernal and the devices performance is they pushed the S4 out the door early in response to the HTC and Sony devices which are real competitors to the S4 and would have soaked up a lot of buyers. That of course is a guess on my part but I believe it to be true. The S3 had no real competitor of the caliber the S4 faces now and I think its more solid firmware out the door has to do with the fact that they had more time to sort it out.
Regardless the one does not have expandable storage and for me thats the end of it since 32 gigs will not carry my payload even if it was all available. I'm unconcerned about fixing the firmware as I fully believe it will happen and soon. My S3 was absolutely without the slightest lag or stutter anywhere in its UI and I fully expect the S4 to get there as well.
In the end I'm glad of the competition heating up because it was starting to look like a two horse race between Apple and Samsung. This year I think Sony came up just slightly short but they did come close this time and got the phone out while it mattered instead of 6 months after its hardware was dated by flagship standards. HTC seems to me to have hit a very comparable offering. Lack of storage makes it a non starter for me and I also avoid devices without a removable battery but other than that it might have been a buy. Things are getting more interesting for shoppers again.