The stinger for CFDs lie in the interest charge on the total value of the holdings!! So as the share increases so too does the interest charge. If you have a loan/margin you are only being charge for the margin portion. Can make a big diff if holding for a while.
(...and to the post above): keep in mind that for shorts you are PAID interest. If you're like me and like a good proportion of shorts to longs to balance off a plane-hits-building scenario, the interest pretty much balances out.
I do have more longs than shorts, particularly during general upswings, but about 40% of my account is in direct shareholding, and ALL of that is long (sometimes I want me some sweet franking) - so my CFD account is usually at about 50% or more short - hence
earning interest.
My average trade duration is about a month. I see CFD's better for daytraders.
My average hold is less than two weeks, but more relevant to interest calculations is how much of your account is generally in an open position. So there's not much difference depending on hold time, just in how much of your account is open long at any one time. For me, since about 50% is long and 50% short in open positions at any one time, my interest is roughly zero.
If you were mostly (or all) long, then it would come down to your exposure over time - how much of the account is in open positions. If you hold one position for a year, that'll cost the same amount of interest as holding a series of 2-weekers, if you always hold a 2-weeker at any time. Then it's the brokerage / spread that counts against changes in holding time, not interest (and in that case always favours slower trading).
(In my very ignorant opinion) I think CFDs are a bit crap for day traders, since the good stuff to day-trade (futs, forex) end up with ugly-ass spreads (but I'm not the best person to talk to about the day-trading side, so take that bit with lots of salt).
But CFDs are good for short-term share trading, becuase the brokerage is decent, the margin is great when the opportunities are there to be taken, and the ability to short is invaluable.