Well you're going to have different gains in USD for it, not in Bitcoin. You may have more or less Bitcoin depending on the price point in which you bought it, but it'll just tell you how much BTC you have. Only difference is going to be when you sell you will make different profit amounts on those two.
That's only really if you're calculating the profit which you make, which you should be if you're a trader.
Yes I agree with this one. Some people like to use blockfolio to track their portfolio or you can use an old fashioned spreadsheet