Depends whether you want lycra blend or 100% merino. That's really your options. Both have their advantages and it's really down to personal preference as to which you prefer.
You use wool interlock practically the same way you would knitted wool, except lycra blend you can normally wash in the machine on a gentle wool wash at 30 or 40C as it won't continue to shrink. It's good on CSP for that reason as it won't continually warp with washing.
100% merino has all the purity you'd expect from just merino. I'd look for wool interlock that's Okeo Tek certified and meets CSPIA regulations or similar standards. I think at a push I prefer 100%, but then lycra blend is quite good for longies and soakers too, soakers especially, because of the stretch and fabric's memory for shape and the ease for washing - you don't have to be so careful with garment washing care with the blend. 100% merino will mis-shape and tend to pile same way merino handspun or yarn would tend to, just because of the fibre type and short staple. if you've got your 100% merino longes or crops, imagine that softness in wool fabric form.
Lycra blend doesn't take dye quite so well in my experience, you can see the lycra in the fabric, but in terms of function and material they both have their merits. I lanolise, handwash, spin in the washing machine and leave to dry flat preferably. Dead easy. Only risk with 100% is they get thrown in a normal wash by mistake, but same with your knitted woolies.