[Xperia Z2 General] One Twelve:
1. no difference, both use the same qualcomm DAC which isn't as good as wolfson but this is what you get. You may get more audio mods to tweak things. Without headphones its m8 all the way.

2. m8 will do that fine and will focus faster than Z2. But its a foreground shooter, if you want to blow things up later you won't be able to do it. landscape shots so long as you don't want to zoom. You have to get up close to your subject with the m8, zooming in after is moot. Camera on m8 is niche and it offers a lot of options, much more than Z2 but you need to know when to use them. I'd say to get the best out of the m8 you need to like photography. SA on Z2 is quite good, so if you don't want to mess around too much the z2 will work better here. Z2 is more a general purpose camera, better rounded which for me means more useful.

What about video ? stabilisation on z2 is very good at stabilising walking, very few do this well. No stabilisation on m8, not even in software which is a major omission. The extra camera seems esoteric right now. Yes, it can be used but whether you conisder it essential is another matter entirely.

Remember good photo is more a function of who is taking the photo than what is used to take the photo. The 12 inches behind the camera are the most important. If you don't care for this then checkout samsung, lg or apple. They cater to the absolutely clueless. Those 3 do not make general purpose cameras, they make good photos in good to average light cameras but struggle with low light.

3. this one is a toss up., rooting is harder on m8 i'm told. but once you're rooted its done. Give specifics of what you are looking for. In some ways the htc has a whole year before the next version drops. with xperia every 6 months there is a flagship. This tends to divert dev attention in the xperia line a bit more often than otherwise. Things still get done though but pace may be slower compared to others. A unique example with xperia is new software features say camera that were offered on Z2 got ported back to older Z lines so long as the hardware was capable. Sony won't do this, but the dev community did it. This takes the sting out of seeing a newer model six months later which has better software features and wishing your model had it. There is cross development across xperia lines where possible.

4. Z2 does better.

One thing you need to be aware of is QA is more or less the same between these two devices, possibly more issues with the m8 as its a bit more complicated to assemble and consequently harder to service. Be prepared to exchange the device in case there are problems when you get it new. So this isn't something you want to buy online. Does not mean it will happen but be prepared in case.

As far as repairs go there is no domestic servicing in India for the Z line, its couriered over to Sweden, m8 i expect goes to Taiwan. Sony claims a 7 business day turn round time which i'm sceptical about. Point being you will have to wait in case of any repairs, won't be as quick (or as cheap) as with other (less attractive) but more easier to repair devices.

If you don't need to pull the trigger yet, there are Z2's arriving which do not include smartband inside the box and will be cheaper.

You could also wait for the Z2L. last year the ZL to me was much better, 5k cheaper, no need for gimmicky water resistant, won't be as attractive but under the hood will be no different to the better looking Z's. Wonder if it will get a replaceable battery this time. The Z's get first pick for dev attention but the mods and dev work come over to the L lines eventually. They were not neglected last year it just took longer.

Same applies with e8 but i don't know if we will see e8 in India.