100% not true. While DL only flies CRJ-200s into DTW, it flies nothing but MD88s to ATL and the larger RJs to MSP

Atlanta (ATL) 903mi
May4 6 55a 9 29a 1263DL 0 6 M88
May1 May10 6 55a 9 31a 1263DL 0 X6 M88
May13 6 55a 9 31a 1263DL 0 123 M88
Apr30 6 55a 9 37a 1263DL 0 Daily M88
Apr7 12 12a 2 54p 2225DL 0 Daily M88
Apr8 Apr30 12 32a 3 14p 2225DL 0 Daily M88
May4 12 36a 3 13p 2225DL 0 6 M88
May1 12 36a 3 15p 2225DL 0 X6 M88
Apr8 Apr30 5 00p 7 43p 1112DL 0 Daily M88
May1 5 17p 7 55p 1112DL 0 X6 M88
Apr7 5 59p 8 40p 1112DL 0 X6 M88

Minneapolis (MSP) 1115mi
Apr13 Apr27 6 30p 8 40p 3327DL 0 6 CR9
Operated By Pinnacle DBA Delta Connection
Apr8 Apr30 6 30p 8 42p 5209DL 0 X6 CR7
Operated By Expressjet DBA Delta Connection
May4 6 40p 8 47p 5857DL 0 6 E75
Operated By Compass DBA Delta Connection
May1 6 40p 8 48p 5857DL 0 X6 E75
Operated By Compass DBA Delta Connection

* Note the "operated by" part.

Personally, I would take the intentional connection in ATL for better planes (and more miles! )