With Vanguard the internal costs are (usually) very close to the same overall. If you are buying small amounts frequently and you don't get free trades through your brokerage, then MFs will be cheaper than buying ETFs.
This recent thread might be of interest to you. Note that the internal expenses and trading fees (commissions), and the bid/ask spread for ETF's aren't the only things to be aware of. ETFs trade at either a discount or a premium to the underlying value of the stocks (NAV) they contain. These premiums/discounts vary all the time, but can can stay on the same side of the NAV line for months (i.e. the ETF could be trading at a premium or a discount for a long time). If you buy shares on one side of the "line" and sell on the other side, you can gain/lose up to 1 percent (typically) of your investment compared to a straight mutual fund purchase/sale.
Also, during the recent "flash crash," some ETFs lost more than 90% of their value even though the stocks they contained had not changed their value appreciably. It only lasted for a very short time (less than a minute IIRC) and was a result of lots of automated trading and a computer glitch or two. I think most of the trades were later cancelled, but it's a little scary nonetheless and highlighted (at least to me) some important differences between these products and a conventional open-end mutual fund. If you go with ETFs it might be best to avoid automatic sell/buy orders just to avoid getting burned if this happens again.
I don't own any ETFs, but I would buy them if there was a particular asset class I wanted that I couldn't adequately cover with low-cost MFs.