I'd give Navionics a call. I've found them very helpful and they will advise the best way to get a good deal. I'm sure someone on here will be interested in your old card which will help toward cost too.
The card usually comes as a micro sd in a sd adapter so can be used in both.
Geographically, you can get either a localised area or the whole of UK. Depends how far you're likely to travel.
In terms of detail options the main thing to consider is whether you want just basic nav charts or the additional option of sonar charts. The sonarcharts are more detailed contours so will give you much more detail to find underwater fishing features so quite useful in that respect. You can also get satellite imagery of land included but how practically useful this is, I'm not convinced.
In all cases you will get 12 months of map updates online as part of the purchase.
Personally, I went for whole of UK as it wasn't that much dearer than a localised area. I also went for the "platinum plus" feature which includes nav and sonar but not sat imagery.
As I say, best give Navionics a call, they are very helpful at guiding you through what options you may prefer.