Well, as others have mentioned, it's all about customer perception. Many people do not realize that there are some ~17 NRTL's in this country. They're still used to the old "UL = USA and CSA = Canada" mentality. What matters to you is how your bottom line will be affected. I'd suggest a comparison to competitor products and see which agencies they all use - is there one that stands out above others? Is it a mixed bag? This will tell you what the end users are used to seeing and most likely what they ask for (and end user survey or general market study may also help). A little education can go a long way if you chose to use an NRTL outside of UL (by showing your end users that there's no difference, from a legal stand point). I had to do this for my company when we switched all of our Canadian Listings from CSA to cUL. There was some initial confusion at first, but now everyone is up to date.
As a compliance engineer, I agree with the different interpretation you'll get from different engineers at UL. However, you see this with other agencies as well, just not as much. If you have a compliance engineer on staff, there's no need to worry about this because he/she will straighten out any mis-interpretations and assumptions (a regulatory compliance engineer is worth his weight in gold!). If you don't have one on staff, assign the certification part of your project to one individual (engineer) and encourage him/her to validate all interpretations and assumptions made by the agency engineers (easier for one person to keep it all organized vs several people). Remember, the agency engineers are experts in the standards and codes, but only YOU are the expert in your product, its design and intended applications. You have to work together with the agencies to prevent waste in the form of unneccessary testing and evaluation (and ultimately cost $$$). I've also found that TUV tends to be very expensive, in some cases more than UL. Good luck.