This is the kind of decision you'll need to make yourself. It does sound like you've already got a decent idea of what the pros and cons of each option are. Regardless, I'll give you my two cents on some things.>the work is mostly experimental and on a topic that I'm not particularly interested in. My expertise is in computational mechanics and that's where I'd like to focus.
Doing a PhD in something you're not interested in is generally not considered a good idea. Especially when both the topic and the methods are not what you want. To get a decent track record you need to be motivated and self-driven, which is difficult when you have no passion. Of course some people can make it work, but a PhD isn't usually lucrative enough to be worth it if you really hate the work.
It may or may not be the case that you can do some computational work on the side, most projects nowadays have some computational element to them. Simultaneously, there is no guarantee that this will be possible. The advisor probably has a specific job he needs done, and if you give the impression that you'd rather be doing something else he might just not take you.>the monthly salary is more than what an engineer with no experience in my country (assuming I could even get a job in this economy) makes in 2 years
I don't know where this position is, but it's worth noting that PhD stipends often don't leave you with a lot of savings opportunities. They're designed to allow you to subsist in that country. So while it's maybe more money, there's a decent chance you'll live hand-to-mouth and come out no richer anyway.
Of course, doing a PhD in another country builds connections that make it easier to get employment there later down the line.>How much will getting a PhD in an experimental field limit my chances at computational jobs?
It won't limit your chances, but it won't improve them by as much as spending an equivalent amount of time and effort on computational work.