Absolutely get someone experienced in carpet laying if you are at all worried about seams. My dad is/was a carpet layer and would always complain about having to go back and fix badly done jobs by the less experienced. Not only do you have to worry about the seams, but because you have large rooms, you need to worry about the carpet being stretched properly! Otherwise you'll be riding the carpet waves.
There are alot of factors that most people aren't aware about when it comes to the art of carpet installation. I'm only aware of them because I happened to hear alot about it, and I still get to talk to carpet layers on jobs that I'm working on. My dad told me that Berber carpets are some of the hardest to seam together ( although you wouldn't know it by looking at the ones he installed

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Even after watching my dad lay carpet over the years, I installed the carpeting in my 3 bedrooms ( used carpet, had to seam it together ). The overall look is good, but you can tell I did it by looking at the seams, and where the carpet meets the hallway carpeting.
So, I guess I'm saying go with quality over saving a buck to save yourself the headache of a job done poorly.