I was leaning towards doing the picture framing, its a bit more work but i think it will look cleaner. I thought i was reading that composits will flex more then wood so people were recommending 12 on center over 16 to solidify it. I am no expert so this could be way off.
I can't say I've noticed any flexing but others may have different experience. A few bucks more in lumber and fasteners to go with 12".
What composite will do is shrink/expand with temperature. Unlike wood, where dimensional changes are related to moisture and will often settle down over time, composite is closer to plastic than wood in my opinion and the changes are temperature related. That's why they have slotted edges and recommend using the hidden fasteners. For areas with end joints, they will open and close if you experience temperature swings. Ours was built in the height of summer so the joints are nice and tight in the summer and open up in the winter when I really don't care.
Also, I think Trex offers a non-slotted board for use around the exposed perimeter. for the inside edge of that board you can use a router or biscuit jointer to make a slot for the fastener. They recommend you build from the outside towards the house so if you have to rip a board it will against the house and be less noticeable.
Last word on temperature. Darker colours get noticeably hotter than lighter ones. Our deck has a grey field with dark brown perimeter and the difference in summer is significant. Depending on where you are and if you have dogs or people who run around in bare feet (the people, not the dogs - I know they have bare feet) that might be important to you.