I'm currently on my way to a new phone in about a week or so and i've narrowed it down to the S5 and M8. Would i be able to register with the m8 first then return it within 15 days to try out the s5 and then finally decide which one i want?
Cause i'm really torn between the two XD
S5 has a better camera (although the M8's pictures aren't BAD, they just aren't as detailed) and water resistance but that's about it. I absolutely adore the M8. Higher internal storage, metal body and slick design, excellent sound quality with the speakers, super fast performance, and Sense 6 is a wonderful Android skin that isn't bloated like Samsung's.
Now to answer your question: yes, but you have 15 days from the initial activation date. If you exchange for the S5 a week in, you still only have a week left in the trial period. It doesn't reset every time you exchange the phone.
The cameras on the M7 and M8 get a lot of hate because they have less pixels than other flagship like the S5 and Z2. I know the pictures aren't as detailed but the colours are good and the exposure is generally fine, plus HDR helps in shots with light and dark areas. The auto focus is very fast, and there's great things like making mini slideshows with Zoe and Duo effects like uFocus that the S5 can't do. If you're sharing to social media or with friends, you'll be fine with the M8. Serious photographers shouldn't be relying on camera phones.