The model is relatively insignificant... it's features you want to think about.
You probably want something with variable speed. Also, keep in mind that the cord-free rechargeable dremels are great for etching, cut-offs, and other chores, but are not the best for drilling holes because of the torque. They can be used for drilling holes, but they are just not as efficient as the higher-torque corded dremels.
The trade-off comes in weight and balance. I have a cordless dremel and it's a lovely tool -- very well balanced in your hand and easy to hold (that's important if you have arthritis, carpel tunnel, or other hand issues). I also have a corded one, and it's more "unwieldy". By itself, it's heavier/bulkier than the cordless. If you do get a corded one, I would HIGHLY recommend getting a Flex-shaft with it. A Flex-shaft is a little easier to hold/control with same amount of torque for drilling. However, you still have a "tail" (the cable from the Flex-shaft back to the dremel tool) that weights down the back end of the tool which... again... can be hard on your hands. A downside to the Flex-shaft is that the inside is made with nylon/plastic parts which will eventually strip out, requiring you to buy a new Flex-Shaft (you can find some do-it-yourself repair hacks on the internet). Anyway, I have an aversion to power tools and water -- the Flex-shaft moves the A/C power source away from the water you're working in (and so does the cordless drill).
So... it's a matter of trade-offs. If you are young with strong hands, go for a variable-speed, corded Dremel, preferably with a Flex-Shaft. If you have hand issues you may want to look at a cordless model and know you are compromising a little on the efficiency of drilling the hole.
Well.... that's my 4 cents anyway