the form factor of the DX8 is very different than the form factor of the DX6i. The 8 is more comfortable than the 6. The old 7 is a little thinner than both but also comfortable.
If you already have a 7, no need to buy a new one. If you have a 6 and money is tight then the 7 might be a good fit. If you can afford it, the 8 is really really nice. The backlight is darn useful.