I actually think intensive classes are much easier; focusing on one subject only is far far easier and you retain a lot more information, provided you actually study (and to my knowledge, ~*~*science*~*~ backs up my opinion on this). Also, Engineering Maths is a six-week "intensive", so you'd have MORE time to study for it than in semester, when you're typically juggling multiple subjects. Just my two cents, but it seems like an absolutely fine option to take it over summer, with lower stress and more continual exposure with fewer large breaks between studying the subject (both due to the doubled number of weekly classes and the fact that you don't have to focus on anything else). I certainly found my Lv 2 summer class a lot easier to conceptualise, and it ran for only two weeks, not six, with an equivalent amount of content to an ordinary biology subject (though it was conceptually simple, I don't think it could be any worse with three times as long to complete a somewhat more complex class).
So, if you don't have space you want to give up in semester two, or want to lighten your load, I reckon summer semester is a perfectly fine option, if you don't mind losing some time from your holidays over the six-week period.