FRONT; Besides the comments about the font, lower "Envious Detailing" so it's centered in the white portion (the way you have "Creating a shine..." on the bottom).
BACK; I like the 2nd one better but it depends on who your customers are. If you only do really high-end cars the 1st one is a better choice. If your bread-and-butter is daily drivers with the occasional high-end, my opinion is the 1st one would scare away potential customers with it's pics. Comments on 2nd "Back"-Lower your name, don't go to the edge of the printable area. I'd delete the word "offering" and go with "High End Detailing at Affordable Prices" or "Professional Detailing at Prices You can Afford". Delete the word "Full", make it "Interior and Exterior Packages Availble". Next line in a smaller font - "or let us customize a package that fits your needs" or "just for you". Capitalize the words "vehicle" and "reconditioning" to keep everything consistent. Try centering your web address and then you can add "Call Now" and your phone # to the left of the car. Overall, I love the photos you've chosen but there's no urgency in your message and that's what advertising is all about. Isn't it alot more expensive to have them design it with your photos, etc.? Alternatives would be OvernightPrints.com (excellent quality) or VistaPrint.com (I've never used them). They both offer postcard templates as well. Please note; I'm just trying to help and the above is just one man's opinion, and you know what they say about opinions. LOL