Full disclaimer: I'm not a knife fighter and don't pretend to be. Also, I don't actually own a sirupati, I'm just bench racing here.
I didn't mean to say that you implied it was fragile, just that you keep saying you'd use a sirupati for light work. I offered the example of the BK9 because that is a knife that most people would say is fit for 'hard' use. As such, if the BK9 is fit for hard use, I suggest that the Sirupati is also. Ultimately I'm suggesting that you don't need to limit the sirupati 'light' duty (in case that was your plan), even if it isn't warranted for 'field' use. Just my opinion of course.
So far as the 15 - 18 inch version, between the two I'd go with the 15 inch. The 18 doesn't have much of a reach advantage IMO. Consider defending yourself from someone with a baseball bat, if you had either the 18 or the 15 you're still well inside their danger zone for the blade to be effective. Once you're well inside their space such that a club is a less effective weapon, the 15 inch sirupati has plenty of blade to do whatever needs to be done. Also, the 15 is easier to carry, easier to hide, and weighs less. Likewise the 18 is on the short side for machete work, and too heavy for that sort of work anyway, so again why not just get the 15 at that point, right?