As a general rule, IWI creates thermal bridges and EWI covers them. This is not always true, particularly at EWI/opening interfaces, mainly windows and doors. These need very careful detailing. The one area that EWI is not so good at is abutting ceiling height loft insulation. If you have a warm roof it is usually possible to abut to the insulation. In either case insulate above ceiling height.
If you live in a semi or terrace you might still want to insulate the party walls with IWI. This assumes that either the houses either side are significantly colder than yours or you want aparticularly high level of insulation yourself.