Hi,
well I can't be 100% unbiased because I already have experience in this small-medium company market (I have worked for 2 NAV partners in the UK). Overall I'd go with Bi4Dynamics because it's more mature and slightly cheaper to license than Jet Reports Enterprise.
What I've seen so far is that Jet Reports Essentials, to be used properly, needs to be learned in so much or even more depth as Excel. Most small companies don't have someone available to do that.
Bi4Dynamics includes 100+ reports pre-built, which can be copied, adapted and adopted by anyone who knows how to use Excel pivot tables and charts.
If you are picking between Bi4Dynamics and Jet Reports Enterprise, then it's fair to say that the Bi4Dynamics solution is more mature as it's been on the market for longer, improving both the back-end and the included report set.
I've also seen that companies where the NAV and SQL permissions were done "cheap & nasty" they had problems because users get access to NAV tables they shouldn't. This is of course not a shortcoming of Jet Reports, rather a degree of tolerance that a SQL based solution does not have, therefore exposing these problems before you go live.
If instead you choose Jet Reports Essentials, just ensure that your budget covers a minimum of training every year, otherwise you'll end up with a collection of reports with workings that are understood only by a minority of Excel gurus.
Nuno Lima