Specs for all three series, WC, BGN, and CTS, are somewhat similar.
All the series have commonalities and differences.
Differences in three series are in several areas, in some areas the trucks and WC are more alike and in others the trucks and BGN cars are more alike.
Engine - All three series run 358 cubic inch (max) engines with 12:1 compression ratios. The Winston Cup cars run an 830 CFM carb, while both the BGN and CTS engines are restricted to a 390 CFM carb. Additionally both the BGN and CTS teams can run a roller cam instead of the flat tappet required in WC. The BGN and WC cars draw air into the carb through the cowl area, the opening and its airbox being closely regulated in size by NASCAR. The CTS engines draw their air from 2 openings in the grill area which feed through 2 three-inch ducts directly to the air cleaner. The size of the grill openings is also regulated.
Chassis - All three series run a 60.5 inch tread width chassis. Wheelbase length is different in all three. The WC cars run a 110" wheebase, the BGN cars a 105" wheelbase and the CTS trucks run a 112" wheelbase.
Weight - Both WC cars and CTS trucks carry a minimum of 3400 pounds without driver. The BGN cars carry a 3300 pound minimum.
Overall Dimensions - Overall width of a WC car or a BGN car is about 74". The trucks run an overall width of 76.5" to 80". Overall lengths depend somewhat on manufacturer, but all three series are around 194" to 203". Height is closely regulated and minimums are established for all three series. The WC cars must be 51" , BGN cars must be 50.5" and the CTS trucks must be 59".
With all these similarities and variances, it is impossible for me to determine which is more like the other. I would think it would depend a great deal, if not almost entirely, on which type track you were discussing. Certainly on the superspeedways it would appear the BGN and WC would more likely be most similar, simpy becuase the aero factors are more closely aligned. On a short track, perhaps the weight and wheelbase of the CTS trucks would make them more similar to the WC cars.