Is the pay commensurate as what you are now, plus some? If you are truly an able and capable programmer, and have interpersonal skills as well, you'll not be a "junior developer" for long. Since you will have taken the position in good faith, even if you do get treated as a (much more) senior person, it will be unlikely you'll get a raise as soon you might end up desiring.
By saying treated as here, I mean you'll probably be given projects and responsibilities than a more genuine junior person would be given. You need to consider for yourself, if you will be getting an attitude because you're not being paid commensurate to all of those responsibilities. If you, right now, consider the pay raise as worth more than a junior, but at least fair to a mid-level, then when you end up being treated as one, you'll already be fairly compensated.
Another thing is, if you take the position as a junior developer to a more senior one, it will be expected for you to not come in displaying a desire (hidden or not) to 'take over' ... even if you feel you could "do the job better" than the current guy, it's not what you will have been hired to do, so don't. At least until you have been in the position long enough to prove your own abilities and aptitude.
As an anecdote, in my current organization, we had a web development team lead leave for another position. We hired a new senior developer/team lead. A couple years later the old lead wanted to come back and we needed his skills (they were already a known quantity as it were.) He was re-hired with the express acknowledgement that he would not be the lead. Things didn't work out so well.
I know this isn't what you are planning, but plans don't last past implementation very well. People refer to positions as junior, mid-level and senior, even I do, but in real life, things are never so cut and dried. Your skills and experience add up over time to being a quality and quantity. If your S&E are beyond junior, than that is what you are. Getting hired as a "junior" person, when you are really a senior person in experience and skills rarely turns out well, unless there is a well thought out basis for your under-positioning.