Since you are asking about that route, I assume you are going through Oklahoma and will be coming into Texas on US-75 at Denison.
Having lived close to 30 years in Dallas, I'll take 75/45 straight thru almost anytime.
As you are probably aware, 6:30 am til 9 am - US-75 from McKinney to near downtown Dallas is slow, often stop and go in many places. There is also some traffic southbound through Allen, Plano and Richardson during the afternoon rush-hour.
Frankly at those times, the straight thru route is usually easier than trying to deal with rush-hour traffic on I-635, especially through Garland and Mesquite. The worst part of US-75/Central Expressway traffic will be north of I-635.
My preferred Dallas radio station for traffic information is AM 1080, KRLD. Every 10 minutes "on the 8's".