Plain and simply, spesh will be a massive help for engineering. Further will literally not help in the slightest (In terms of the maths you do; maybe Trig & Geometry will be useful revision, but hopefully you already know the volume of a cube etc..). So ignoring what you're 'capable of', you should without a doubt do spesh over further.
As for difficulty, I can guarantee it would be easier to get ~ 33 in spesh than 45 in further. In spesh 33 would be achieved with, at lowest a C+ and B, where in another case maybe a B+ and B may get that (depends on a lot of variables). 45 in Further is A+ for everything, top rank, with losing, at max, a couple of marks in the exam over all. Very, very hard, not because further is conceptually difficult, but because it is hard for humans to be this perfect under pressure and in limited time.
Another thing to consider, studying for methods & specialist is like studying for a single subject. They compliment each other as they cover exactly the same areas of mathematics but to different difficulty levels, so this essentially reduces the workload - as oppose to studying two subjects of the same difficulty as spesh and methods that are unrelated. Further is nothing like methods, and so they would not compliment each other thus meaning essentially more study (If you're looking for 45 raw anyway, otherwise you could get away with not much study for further). Also, doing spesh will make you much better at methods as well, so even if you don't go as well in spesh for what ever reason, you will go better in methods regardless.