Lukaku has surprised me in the last 18 months or so. As a teenager he was always impressive for his age (and a true FM legend to boot), but I had always felt this owed to his well developed physical attributes. Once he started playing in the PL he was quite underwhelming for a long while - which I felt was because he had lost the advantages of size and power that he had enjoyed.
In the last 18 months or so he has definitely proved me wrong, and shown that he has the technical ability and attributes to succeed at a high level, and was not simply an overdeveloped youngster. If it were between him and Kane then I would be torn but perhaps side with Kane for the English factor - however with Spurs looking to comfortably finish above us in the league I am not sure if this summer is the right time for Kane to move, and there is always the problem of having to deal with Levy.
Martial and Lukaku have the makings of 2/3 of a great attack for years to come. Perhaps Mahrez for the RW position...