Whichever you do, you *should* spend a year abroad (there are sometimes some instances where you don't, but generally it's a yes).
I think generally more people tend to do the teaching assistantship/work placement - but as places are limited for study abroad that's probably common sense. I did a gap year as well, so sometimes it's a bit odd to think that I'll graduate when I'm 23, but I"m having a good time so it doesn't bother me.
What I would say if you were thinking about doing the study abroad option - it's intense. I have a friend currently at Sciences Po in Paris and the feeling I get from her is that if you want to do it, you not only need to have good enough grades, but you have to be motivated enough to cope with *insert your modern language choice here* all day every day and doing all your work in it too. What put me off it was the fact that it basically *is* your Junior Honours year - I'd much prefer to do my Junior Honours year in a familiar environment that I'm used to, rather than having to do it abroad - especially considering the first few weeks of settling in/down.
Teaching abroad is pretty much the way to go (not that I'm biased or anything). You get a year out from studying (apart from the year abroad project), you're being immersed in the language/culture, you're being paid to work 12 hours a week, and I've only been here for 2 months, but already I feel like my confidence and independence has improved dramatically, as well as my French.
In the end it's really up to you, but I wouldn't let the thought of "length" of degree make your decision. Right now I'm actually quite enjoying the fact that I get to go back next year and still have 2 more years of studying in St Andrews
Hope that helps!
Let me know if you have any more questions =)