I have never owned or used an older Dremel, but I do have one of the 4000 series (corded version) now. To be honest I have been quite disappointed, but maybe I was just expecting too much from it. I do realize it is a hobby tool and not for actually doing much work, but even for a hobby tool I was expecting more. I do a lot of small projects that require abrasive cutting wheels and sanding discs and the Dremel just seems gutless and also very easily overheated.
I have the flex shaft attachment for the Dremel as well and just hooking it up stresses the motor enough to drop the no-load rpms by at least half before I even start doing anything else. It gets hot within 2-3 minutes no matter how easy I take it without the flex shaft, so the flex shaft is for very quick tasks only. I actually thought I had a dry bearing when I first used my Dremel because it got hot so quickly, so I took it all apart and replaced the lube in the bearings with better stuff and it is still the same. As a result I now just use all my Dremel bits in my larger Rotozip and I couldn't be happier.
I rarely need to get into tight areas, so the extra size of the Rotozip is not an issue. It never even gets warm, it can run for a half hour or more without issue under heavy cutting loads, and it barely slows down at all when cutting or grinding something. I now only really bring out the Dremel when I need to use the flex shaft or right angle adapter to get into an impossible location. That has only happened a few times in the last year or so. With that amount of usage the Dremel should serve me well for many years as long as I never let it get too warm, and for the situations where a Dremel is needed there is usually no other way to accomplish the task, so it is sort of a necessary evil.
The newer Dremel heat issue/switch burning out issue is a well known issue online, so you will just have to baby it if you want it to last a long time. If I had the older one I would try to fix that one instead of buying the new one just based on the much better reputation the older models have.
If my Dremel ever burns or wears out I will definitely not be getting another one. I will gladly pony up the extra bucks for something like a Mastercarver or a Foredom flex shaft carver/grinder instead. They are a lot more versatile. Even Harbor Freight's knock-off version is probably a better buy, more powerful, and more useful than my Dremel.