This was a decision that I struggled with before buying my Dremel and, in the end, I think I got it wrong, at least from the purely technical standpoint.
Let’s begin with the obvious – the cord ties you down to an outlet, while the cordless tool lets you roam. Even if you are stationary, the cord limits your range of motion, dictates how you grip the tool, and requires you to be constantly aware of its position. On the flip side, the battery-powered tools are typically weaker than corded ones, and the batteries periodically run out of juice and need to be charged.
I read a bunch of professional reviews and ended up with the corded Dremel 4000. Power! Uninterrupted work! I feel like a pro and my father-in-law approves of my choice. Great! Except – I’ve been working mostly with wood so far, so the fact that the corded tools cut through a 1/4″ (~6.5mm) steel bolt 2 seconds faster than cordless ones (or something like that) hasn’t been relevant at all. For the kind of work that I’ve been doing – small table-top projects – I also do not require hours of uninterrupted tool work, as most of them are finished in 45-60 minutes. This means that if I started with a fully-charged battery, I’d still have plenty of juice left at the end. On the other hand, the cord has been restrictive and inconvenient.
So, all things being equal, as an amateur Dremel user I really should have gone for the cordless version. However – and this is a big one, at least for me – things are not all equal and one thing in particular that isn’t is the price. Buying a very similar kit with the cordless Dremel 8220 would have cost me roughly €45 (~$60) more than with the corded Dremel 4000. So, while it is unfortunate that my rotary tool is not cordless, I am happy with the decision.
In the end, what’s my advice to other amateur Dremel users? If money is not tight, get the cordless tool. The discomfort and inconvenience you’ll avoid are worth the extra cost, and the things you give up (extra power and longevity) probably wouldn’t have ever come in handy anyway.
(A point to note is that the price comparison above is done between two top-of-the-line tools, as of this writing: the 4000 and the 8200. While it’s nice to feel like you are buying the latest and greatest, the simple truth is that at the amateur level we simply don’t need it. So, a second-tier cordless tool is probably the best option for those on a budget.)