I have both a 305 and a 705.
I keep the 305 at our vacation home in Arizona. The 305 has a number of attractive features which make it a favourite for me.
Firstly, it is smaller and therefor takes up a lot less real estate on the handlebar than the 305 does.
The 705 has, in my view, two main selling points. You use a power meter and want to be able to use just one measuring device ( I have used the 705 with my power tap and it works very well for this - though my PT is now listed for sale). The other thing the 705 excels at is mapping, if you need this, and I am not convinced that most of us do.
The 305, only in my experience, updates on screen information much faster than does the 705 ( grade and elevation data ) and is MUCH smaller.
Having used both - my recommendation, FWIW, is that unless you are using a power meter, or absolutely NEED the mapping, get either the 305 or the new 500.
I don't have the choice really. I have 11 bikes. I do not feel like replacing the mounts on 11 bikes. The 305 and 705 use the same mounts.