You have to evaluate your own needs and determine what works best for you. Unlike B6, DL does a good job covering most of the country. But, so do AA and UA. But, if you only fly to those limited amount of cities, B6 might do the trick. Here are a few data points:

• Upgrading to C+ when you fly to SFO can be a challenge if you aren't a PM/DM who books in advance. I can't speak for the other cities, since my view of the country is opposite of you and I would approach them via SEA/LAX/SLC instead of DTW/JFK/ATL.
• Upgrading on PWM - JFK was easy, but DL at JFK is garbage IMO. Terrible ops.
• I like lounges, but the quality of Sky Clubs is very different. I mostly fly out of SEA and it has a really nice club and the food is good. SFO is similar to SEA, but the clubs at ATL are hit and miss and the food at the B lounge was garbage during my last visit. If they are really worth it to you, then go for it. If not, stick with B6.
• DL has 31" in Y and 34" in C+. IIRC, C+ is like regular Y in B6, but B6 is going to be moving the seats closer together.
• I like DL, but one reason is that DL gives me what I need (SJC - SEA and SJC - small east coast/inter-mountain west cities in 1 connection). You have to figure out what's best for you.