As you said, Mi2S upgrade is quite minor for those who have Mi2 device, a slightly faster processor (1.7 Ghz) and better camera (f2.2, 13 Megapixel) for 32 GB model only.
For those who wanted a new phone NOW, yeah they definitely should go for it but I still recommend waiting till Mi3 release which would be around October... Although by the time we get it, it would no longer be October T_T.
Based on the rumors, I think Mi3 will be something like this:
1) Bigger screen (It might be bad news for some people who want smaller device)
2) Water Resistant (f**k Yeah)
3) NFC (Cool)
4) LTE (Not much of LTE carrier in my country but still nice to have)
5) The usual CPU and Camera upgrade ))
6) Higher battery capacity
As for myself, I hope there will be some additional improvements which are as below (More than just hardware):
1) Wireless Charging
2) A slimmer design + better phone design and build quality (Personally, I love the build quality of HTC phone so much)
3) A stronger vibration motor that adjusts the vibration strength based on the surrounding (The current one does not vibrate strong enough in outdoors)
4) A louder phone speaker
5) Globalization (Very unlikely to be achieved within such a short time frame)
6) Xiaomi team to communicate and works together with international developer team such as miuiandroid (Very unlikely too, considering their current response...)