Your best bet is to crunch some numbers.
How many GB would you like? Keep in mind that the iPhone 6 is a 4G phone and on top of that is a data hog. Not only is the phone, but the user could be too and being as this is your first smartphone I would be cautious getting a plan that doesn't have enough GB.
Would it be on an individual plan or the more everything plan?
It might be cheaper for you to sign a 2 year contract, with the device payment plan you'll save $15 off of the $40 access line fee.
For instance if you do a 2 year More Everything 2GB plan - you pay $40 for the 2GB each month PLUS you pay $40 for the smartphone line access fee and for the other phones/devices on your plan. The phone is then $199 + tax. In 2 years you'd spend $2,119
On the device payment plan you'll pay $40 for the 2GB each month PLUS $25 for the line access fee, but then you pay $649.99 for the phone in installments. In 2 years you'd spend $2,210
With these numbers it would be cheaper for you to do a 2 year plan, but then you'd be locked into a contract.