If I wanted to explain in simple terms the difference between Marketing and Sales, I would use the following trivial example:
Marketing is getting them to call you. Sales is you calling them.
Joking aside, its actually pretty accurate, as Marketing is about raising your company or product profile to the extent that people out there in the market place have heard about you, while Sales is about the direct approach, for example calling up a prospective customer and actively telling them about your company and product.
So which is more important, Marketing or Sales? Actually they both compliment each other, so you really need both. Marketing make it easier for Sales to get that all important first meeting, as prospects are more likely to agree to meet with a company that they have heard good things about in the market place. Once that threshold is crossed, Sales then need to deliver a compelling and relevant demo pitch, and establish trust and confidence in the prospective customer that they become comfortable enough to sign off on a new purchase order.
So where does Engineering fit into this mix? Well as an Engineering leader, you need to work closely with both Marketing and Sales. Firstly you need to deliver a compelling product that is interesting to demo to prospects, then you need to work with Marketing on how to position that in the market place, for example by comparing your product strengths and weaknesses with your rivals, and building marketing stories around that.
On the Sales front, Engineering has to operate in a "sales enablement" role, by providing Sales with product training, product demos, and related collateral to enable them to build exciting demo pitches. Furthermore, you may find yourself actively presenting some of those pitches depending on your presentation skills, especially if Sales need help with demonstrating to technical audiences (as engineers can communicate more easily with other engineers).
You must not forget that your Engineering team cannot succeed without Marketing and Sales bringing in the money to keep the lights on, while you must also remind them that without Engineering, there would be nothing available for them to sell! It is a symbiotic relationship, that should be built on mutual respect.