the lack of current in lake fishing compared to ocean fishing is a huge variable when determining what line to reach certain depths. i live and fish in boston harbor. when the current is strong i fish a 525 grain sink tip on a floating running line. it will get the fly deep enough. in not so fast moving water i go to a 325 grain sink tip on an intermediate running line. in both of those situations i use the moving water to create the action on th fly. however, during slack tide i use an intermediate tip on a floating running line. like your friend accurately points out, the floating running line imparts more action on the fly. since in lakes it's slack tide all the time, albeit the depth may be substantial, i would think that your considerations are quite different.. if your target area is 15' or so that's akin to "slack tide". but what if you need to get to 50'? in such a case the floating running line will be too much of a hindrance in getting the fly down. hence the trade offs begin. one small tip on action on the fly. learn to tie the loop knot if you do not already do so. the difference in the action on the fly is startling.