I bought both of my S5s for $99 bux, you will not get the S6 for anything close to that for about 2 years.
The differences in technology between flagship phones is so small as to not making much difference.
The S5 is a very good solid performer and will continue to be for years yet.
I have a Galaxy S Fascinate (Verizon's name) which has been registered for 4+ years and it is still doing a very good job. Yup, it is super slow compared to the S5 but it is just as fast as the S5 for telco and SMS jobs.... The camera is slower, but the detail in it is just as good as the S5 when viewed on the phones, or when posted on forums in 400x600 format. You cannot tell which phone the shot was taken on, the 'S' or the 'S5'.
Host the pix on a web file host though, and there is a lot of difference between 2m pixels and 16 mPixels. but for forum use, I can't tell any difference at all.
so, the bottom line is, the S5 will do you a very good job for as long as your pocket itch can stand to use it. I see a lot of folks who can't sit still until they empty their wallets on a new toy...
a good friend just spent $33,000 on a new GL1800 Goldwing.
I just spent $7,000 on a "new to me" GL1800 Goldwing.
Sitting side by side, you cannot tell which one is the older one.... wallet itch is what it is.