I think this interest rate thing is a bit of a furphy. I would like to point out 2 things.
1. Even if you are using margin lending over an index fund, you are EFFECTIVELY paying interest on the entire portfolio valuation at any point in time. Why? Consider two scenarios:
a) you borrow $50k at 7% p.a. and contribute $50k for a $100k portfolio. At any point in time, you could sell your 50% of the portfolio and stick it in the bank and recieve the ONCR (well in theory). So EFFECTIVELY, you are paying the ONCR on YOUR contributed funds anyway.
b) You contribute $10k, and use marketech to buy the index, paying (ONCR+1%)*100k.
Probably not a hell of a lot of difference.
2. The real reason that an investor (as opposed to a trader) would use margin lending is simply because the capital gains tax is halved after holding for >12 months. CFD's will not have the CGT halved, regardless of time held.
[This post is not intended to be used as financial advice, and may contain errors]