Hello,
I am student and have to ride 7-8miles a day most of the part on road and around 0.5 mile offroad.
I tried mountain bike it was too exausting for me and road bike was little uncomfortable especially off road.
I decided to go with comfort/hybrid/dual-sport bike.
I need durable bike which will need less effort to ride, and low maintenance.

Polaris Sportsman Men's Comfort Bike
Polaris Sportsman 26" Mens Comfort Bike - Walmart.com

Hyper SpinFusion Adult Fitness Bike
700C Hyper Spinfusion Adult Hybrid Bike - Walmart.com

NEXT Avalon Men's Comfort Bike with Full Suspension (also getting same one used for $50)
Avalon Men's 26" Cruiser Bike - Walmart.com

700c Schwinn Men's DSB Dual Sport Bike
Purchase the Schwinn 700c Road Bike, DSB Dual Sport for less at Walmart.com. Save money. Live better.

Diamondback Adult Wildwood Citi Classic Comfort Bike (getting used one too, 2008 model for $140 on community mentioned below)
Diamondback Wildwood Citi | DICK'S Sporting Goods

there is some bike on sell on local community. They do have some used bike on sale and some for free.
R Community Bikes, Bikes for Sale. Can?t Keep Site Up to Date!

I am not sure from where should I buy or which one?
used one or new one?

I would get the DB Wildwood Citi. This is a steel frame but better quality than Walmart bikes.

Bikes at Walmart come in one size and are very heavy.

Buy a dealer quality bike in your size that is lighter in weight and you'll be a lot happier.

I'm not sure where you live and you don't mention a budget, but you can find heavily discounted 2013 Fuji Absolutes (flat bar road bike) and Traverse (Dual Sport) models remaining in stock at Performance Bikes (they're all over the place). Also, you mention one of the bikes at Dicks Sporting Goods. A friend of mine recently picked up a Nishiki Manitoba hybrid/flat bar road bike from Dicks (Nishiki is now the "house brand" for Dicks) at a good price - he's very happy with it and it seems to be a good value for what you get. Aluminum frame and no suspension fork. You mention you want a bike that takes less effort to ride, and I might recommend a bike without a suspension fork even though you mention a dual sport bike as an option. You can purchase one with a lockout option, but the price starts to rise.

Good luck with the search!

Of your choices, the Diamondback would be the easy choice. MHO

hey guys bought diamondback Edgewood (aluminum frame, 700cx40 tires) just for $107

For that price, if it is in good shape, it was a good buy!

it's totally new one, might have been ridden few miles may be.
but it might need some tunning, I guess
Since chain sleeps some times, may be because it wasn't assembled properly
Will ask LBS or bike community

Diamondback is a good value bike. Its a division of Raleigh. Sweet price for a beginner bike.