I'd pick Further, but please don't underestimate the difficulty of a subject. Also, no if you lose a few marks, it does not stuff up your SS. Thats a lie that gets spread around very quickly every year. =/
Depends on what you mean by 'stuff up'.
While further is easier, a lot of people underestimate the competition and not put as much effort into it that they could, and as a result drop their study score a little.
On the other hand, if you do spesh it will help a lot for engineering later on (you can still do the unit at uni that covers spesh if you don't do it in VCE though). It's just that you're going to need to be able to do higher level maths in engineering later on, so doing spesh is advantageous. With that being said you shouldn't do it for the sake of doing it. You sound like you're doing alright in methods, if you're willing to put in that extra effort for spesh then it will pay off in the end. Even if you do slightly above average in spesh, the scaling will nearly compensate for what'd you'd get in Further. If you get a 30-35 in spesh it scales to around 42-47, which you might be able to get in Further. But then you could have done spesh and learnt a lot more for what you're going to use later on, and well you could even get higher than that in spesh and have it scale about 50. I'm not suggesting you do a subject for scaling though, just comparing what you could get as a result. As you get to the higher end of scores in further it becomes a lot more competitive and harder to gain those few extra study scores.
Spesh will probably be hard, but if you put your mind to it to put in a lot of effort, it'll pay dividends in the end. If you think you can cope with spesh, then go for it. As Mr Keshy has said, you can start off by giving spesh a shot, and if it is too hard at the start of the year, then drop it and pick up further, which would be easy to catch up on.