Dremels and other hobby tools are restricted to 1/8" collets and are good for detailed work but are not really cut out for large removal of wood or for use with larger specialized bits (for example some of the tricut bits Harry is refering to). You are better off buying a die grinder with a 1/4" collet. Die grinders are much more powerful and are intended for industrial use including grinding metal, so they typically don't bog down or burn out under extended use with wood. One thing that I consider almost a requirement is to get a die grinder with a variable speed adjustment - normally from about 10K RPM to 30K RPM. This allows you to adjust speeds depending on the work and the bit.
If you intend to buy a die grinder here are a couple of things to consider:
1) Do you buy electric or air-powered? Both have strengths and weaknesses. Electric are more expensive and have to be handled with care in wet environments. Air are much cheaper and can be used around water without problems, but they require that you own an air compressor.
2) Do you buy straight or 90 degree angle? Depending on the bits you use and how you approach your trees you will ultimately prefer one or the other. You want to cut with the grain of the wood so you will either hold your tool parallel to the deadwood grain (90 degree) or perpendicular to the deadwood grain (straight). I personally prefer straight because I do smaller and less frequent work - I think that 90 degree is better for heavy and frequent work.
Electric variable speed die grinders with 1/4" collets can be bought for $170 - $250. Two good brands are Makita and Flex. Fixed speed electric die grinders can be gotten for a lot less - in some cases $30-$40 - so if you don't mind switching bits and tools regularly, you can buy a few single speed grinders for the price of a single variable speed tool.
There are many options for air-powered variable speed die grinders, since these tend to be industrial auto shop type tools. Believe it or not, you can pick these up for under $30, though of course it means you own a compressor. One nice thing about going the airtool/compressor route is that once you own a compressor you can also pick up other cheap airtools that offer great applications for bonsai - like a detail sand blaster.