Trunk + rack is probably more versatile in the long run -- perhaps at some point in the future you might want to use panniers, and you won't need to worry about additional hardware.
That said, I have the Detours High Tail ULP and it has worked very well for me. Had a few early episodes where it was probably mounted a bit too low, and it brushed against my rear tire coming out of a bump (there are skid marks on the bottom to show for it!), but I have had no problems since I moved it up.
Things I put in it: wallet, cell phone, sunscreen, chapstick, lock(s), key(s), tire levers, spare tubes, bike tools, snacks, camera, asthma meds. It carries a good amount of stuff but I do have to decide what's essential.
There are times when I wish I had a rack on my bike and there are times when I'm glad I don't have one. Go figure.
2014 Bobbin Bramble / Brooks B67
2008 Rodriguez Rainier Mirage / Terry Butterfly Tri Gel
2007 Dahon Speed Pro TT / Biologic Velvet