In an attempt to actually answer the extremely good question as opposed to some of the perfectly good personal opinions:
As some of the comments have alluded to you are referring to Pre-sales Engineers who are this lovely breed of technically savvy, personable (mostly) and engaging characters who can articulate a technical message, marry it to a business requirement/message and do it convincingly.
I have been working in this capacity for 6+ years now and have been through many iterations of compensation, some of which were better than others. They were:
1. Per-sale based compensation - not so good as you're eating into the sales person's comp, and they don't like it. It also incentivises you to act more and more like a sales person. Not so good either.
2. Qualified pre-sales visits compensation - generally "how many pre-sales calls did you make". The goal is to measure the ability to generate new business. Not so good, as it's very difficult to quantify and track, and the general pattern of behaviour is to just have stacks of meetings without providing any quality.
3. Quarterly/Annual Revenue based - this has been the most successful in my experience. Success is measured on overall revenue generation of the sales organization (of which this kind of person is a part of) rather than individual sales based commission. Commission is generally a fixed amount per-quarter based on attaining revenue figures or % growth thereof over previous years/periods. This is good as it tends to remove the person a step or two back from chasing individual sales, and then bickering over the commission for each one. The fixed comission amount (say 15% of gross annually or something) coupled with the quarterly revenue targets creates a more team based focus for everyone to assist in the success of the venture.
As for the trusted advisor vs. sales debate - the sad truth is that no matter who you are, if you're asked to sell a single product/suite (instead of solutions) you're going to lose a little bit of your trusted advisor status as you're only pushing that single product rather than considering the larger picture/industry solutions.
I hope this helps, PM me if you'd like to discuss further as I have had exposure to a fair bit of this type of situation and may (or may not!) shed some light to help you come up with something that works for your company.