Lucy wins.
Lucy summons Gemini to copy Wendy and there you have it, no need to expound on the result.
If anybody is going to argue regarding mp difference, please recall all the way back to OS arc. Wendy can't barely hold on healing 2 people, Lucy already had feats (summon 5 spirits simultaneously, force gate, urano) to show she has more mp.
Dragon force is a spell enhancing the abilities of the user and power, not to enlargen pool/source of magic of the user. Too much credit is being given to DS magic for nuking and all that, but CS magic is the most versatile one.
No feats to show that Gemini is capable of copying wendy?? These vs. threads are all theoretical based on the given the capablilities of the characters and restrictions. I don't think mashima will ever make a copy of wendy, but we all know they could.
And even without copying wendy, marlin gemini + any spirits other than the comedic ones (taurus, virgo, cancer) would give wendy a hard time, even if wendy got the speed and strength from df.