To answer this question fr yourself, you need to start thinking about how much a fixed spread costs you in comparison to what a floating spread can cost you coupled with the commission.
For example, with a fixed spread, it's easy to calculate that for a 10,000 unit trade on the EUR/USD will cost you $1 per pip right? If the spread is always 2 pips, (A rather decent fixed spread) it will always cost you a total of $2 to open and later, close that trade. With variable set ups on STP/ECN/DMA processors, you may get a commission free spread on some pairs as low as 1 pip but also see the spread jump higher at times of low liquidity or who knows. All I'm saying is that here, you have to simply figure out what the spread is costing you.
In comparison, a commission for each trade can save you loads on the spread but add to the cost as well of course. Again, thinking of 10,000 unit trades, we'll figure it out for the EUR/USD pair.
Some broker usually charges $7 per lot per round turn so a 10,000 unit trade on the EUR/USD will cost $0.70 cents from start to finish, ($.35 to open the trade and $0.35 to close it) plus the cost of the spread. At times, I have seen zero spread or even open with immediate green on Divisa broker's feed but generally, I would peg it at about a 0.4 average spread, often providing me with trades that execute on a 0.3 spread amount. On a 100,000 unit trade, the spread then costs me $4.00 for a 0.4 spread and the commission another $7.0 for a total of $11.00 to trade 100,000 units.
So, for fixed spread, the trade is
Spread=cost
For commissioned brokers, the trade is
spread+commission=cost
None of this of course takes slippage into account which is another whole ball of wax.
not to mention Trading Rebate can be an influence factor too..
Found this chart comparison, they have a spread+commission-rebate comparisons.
Comment anyone.