Thank you all for all the replies!
After long deliberations, I have decided to save my money and hence decided that I will likely get the iPhone 6 64gb.
Like some posters said, it is probably wiser to use the $350 saved to obtain a better phone in 2 years time.
I guess if I find the performance of the iPhone 5s satisfactory at the moment, there is no need for me to get the latest phone.
Additionally, when the iPhone 7 comes out this year, I will not feel so disappointed that I spent so much money on the 6s..
OP are you upgrading from the 5s to a 6?
The 5s and 6 are already similar in performance. Is performance your main issue upgrading?
Put another way, the 6s is a bigger upgrade from the 6, than the 6 is from the 5s, so if the extra 350$ is a big deal for you, what are you spending to upgrade the 5s?
Just be sure you understand what each generation brings. The 6 is mostly a larger screen, larger battery and Apple Pay.
It's a very weak upgrade processor wise from the 5s, with the exact same ram.
Just be sure you know why you're upgrading - if the processor and future proofing is your biggest concern, and you have a 5s, getting a 6 is a very lateral "upgrade". I would just hold on to that and wait until the 7. If you have a 4s or a 5 then a 6 makes much more sense. (Unless you really want the screen estate and if I missed that my apologies)