High would be considered better because you know there's a market for your niche and people are searching for that particular keyword.

In my opinion i would go for the low competition ones because it is easier to rank, i would made a keyword research first to see how much traffic can that keyword bring.

It is hard to rank for high competition keywords. However I am guessing you are using google Keyword tool and their measurement for competition is not entirely accurate and not specific enough.

The best way to determine competition once you have confirmed a demand is to cross reference with 2 or 3 tools such as Traffic Travis GKT and niche Finder this way you can see more accurately exactly how many backlinks, age, Page Rank etc etc your competition has to more accurately evaluate if it is worth going after.

I will go with high competition keyword. because i want to know where my site is landing with the other competitor's site.

I would like to say that if you want to get fast results in search engine so, you can choose low or medium competition keywords and if you want to wait for ranking then you can choose high competition keywords and I always prefer wordtracker keyword tool for keywords research.

I'd go for medium competition for the site and low competition for targetting the articles on my site. Lower competition tends to get fewer visitor thus not increasing your sales, but higher competition is harder to rank but worth the effort if it's bringing you customers to your site.