I guess it depends on how much customizations you have made in the old site. Upgrading can be pretty straight-forward. However, based on my own experiences - starting from scratch might give you a benefit since you can then "do it right" from the beginning. In this case you might want to consider updating to MVC and also latest version of EPI with Nuget.... You can also export content from the old site and import into the new.

Depends on code base size and what customer expects from the upgrade. Recently had a case when we actually decided to go for greenfield (start from scratch) instead of upgrade just because code base was not big enopugh and customer expected to have latest and greatest also from editor perspective. Which may not be the case when doing "just" upgrade (think about blocks and content areas for instance). If you just update crappy code base to latest EPiServer - it still will be crappy web site running on latest EPiServer.

 Some question

1. how big is the code base

2.web forms/mvc

3.how big the content (pages)

4.do you use composer

5.how many images you have

i just finished 3 months of upgrade from 6 to 7.5 because our 3 sites are huge with tons of media.

so it depends. also we are "stuck" now with the web forms.

1. how big is the code base
On the initial VS check it looks a decent size, 20 projects.

2.web forms/mvc
Currently web forms.

3.how big the content (pages)
Not sure, still waiting for the database so i can run the project up.

4.do you use composer
Never used it before?

5.how many images you have
Again still not sure at this stage, perhaps tomorrow.

Hopefullly this helps.