The Northants ultra was my first, and since there seems to be a lot of interest in it, I will make a couple of observations.
It is a very picturesque event and beginner friendly. The start and finish line is in the grounds of Lamport Hall, and the route takes you round a circuit of rural Northants. You soon see why it is called "The rose of the Shires". The course is mainly tails, bridal ways and fields, but with some country lanes. The time limit is generous, so you can work at a slow pace and its not the end of the world if you take a detour.
This event has some big hills and requires some navigation, which I consider to be a good thing in a first ultra. It will force you to develop the tactics and skills that you will need for longer events, such as matching run walk strategy to terrain, map reading, and managing your food/liquid requirements on the go. If you picked a flat 30 mile event, then you would have to go through that learning curve at a later time in a longer event.