It would be better to buy the investment property with cash if you have it. This is because the loan will lower your profit percentage on the property as compared on how it would have been if you had invested in cash. You will also have challenges in selling/lending the property if acquired through loan, since you will want to be on the higher side to recover what you paid for.
The way I see it is if you have the cash do it. If you get a loan then you end up paying more for it. If you get a loan and make payments on time or early then it looks good for your credit but if you get 6 then your credit must be good because my husband and I just bought a house in April and I think ours was 5 or 5.5 and our credit is great!
For an investment property it's usually better to play with someone else's money. At that percentage rate, if you can flip it quickly the profit loss would be minimal. If you can't flip it you can walk away and let the lender foreclose and take it off your hands with a minimal loss ( except for a credit hit, of course).