D4F it seems try to offer a system very similar to trading in the futures market proper. In that respect they offer stand alone stop and limit orders which other sb firms don't.
Remember that stops can be used to enter a trade as well as to exit. This is a very useful facility if you want to go long on a breakout for example. The fact that stop orders are not attached to a working order is very much a plus as far as I am concerned. With
IG you can't enter a stop on it's own to go long in a strong market or short in a weak market, you need to rely on a market order, hesitate and you might miss your entry by 20 pts!
In my opinion if you want to trade properly, cut out the middle man and trade futures directly. You can trade E-mini S&P500 futures realistically with as little as £3000. If that's a bit much to start with then D4F offers the closest experience to trading futures. If you just want the occasional gamble then go with IG or City Index. They are all bookies after all but D4F tries to offer half way sensible odds.