From what I've heard, I'd say it's probably seen as better than just the standard 3 years since you have that extra international exposure. However, it may well be harder since once you return from abroad then you would have a sudden increase in workload.
From what I've seen, people who have done a year abroad tended to get lower marks than everyone else on average although there may be other factors involved. You should bear this in mind though since ultimately your degree classification is the most important factor with job applications etc.