Being in the US, the Steam store is cheaper during some sales.
For example, the WTCC 2015 pack cost 899 vRP, or $9.66. During the current Steam Sale, I can get the same pack for $4.49...the non-sale price is still cheaper at $8.99. The Audi TT Cup pack is another example. It cost 499 vRP, or $5.49, while the everyday Steam price is $4.99.
I'd much rather my money go directly to Raceroom / S3 without Steam getting their cut; however, its hard to justify not saving over $5 if I buy both from Steam.