Just wanted to chime in and add my experience with both providers.
To be clear, Yext is more expensive. It also is a direct feed into more listings. If you are trying to get listings published quickly and need to show results (or reports) to a client, Yext would be a better way to go. As a subscriber, you can also give your clients discounts from the Yext pricing page, which can help add to your overall total commitment from the client. As far as data being removed, the additional data that is added via Yext's api is no longer valid or displayed once you stop paying (they call it no longer a "trusted" listing). I have never had one revert back to a pre listing status. Yext uses one of the many large databases: Factual. Chances are that by submitting to factual the other data aggregators will pick up your listing with time, even if you have not submitted to them.
Moz local is a great resource for companies that have the time to wait. The data aggregators they submit to take time to publish the listing. Acxiom, Nuestar and the others can sometimes make it confusing or even very expensive to list on their data networks. Acxiom is one of the stronger databases, having been known to be a direct feed to Google.
My advice:
If you can afford to wait the 2-3 months to have the listing start to appear, then use Moz. The time period you have to wait is not necessarily the fault of Moz, as even if you submit directly to Acxiom yourself it will take 2-3 months to go live. Ultimately, your listing will appear on the other citation sites, it will just take longer to do so.
If you are looking for an additional source of income and are a great salesman, use Yext. Yext has a much better interface of showing how many listings are incorrect, (even if many of the ones that are listed are from their own proprietary citation sites. ). This makes it an easier sale to your clients, and also easier for you as an agency to get an idea of how bad your clients current citation status really is. Yext will get your listings posted very quickly, and give you a place to link to off of your main domain, giving your clients additional sources of validation very quickly. Some of the sites they submit to are a bit dinky, but hey, a link and validation source is just that. Pricing is higher, but I think its fair considering the speed and extras you get for the money.
AND... the elephant in the room is of course.. do it all yourself! This will offer the most control over all your listings and control over who and what to submit where. You will lose out on some of the other benefits of MOZ (cheaper cost vs your time and long wait) and YEXT (faster speed, lots of directories but higher cost). It's worth noting that some of the YEXT directories that they are partnered with you cannot actually submit to, as they are either created by YEXT or locked so that you have to use their service to submit your listing there.