Although both riggings work, I prefer a fixed mag slinky (25-40 BBs of the small stuff) for side-drifting. A slider works great for boondogging, because of the direct contact with the bait. But, a side-drifted presentation will have a slight bow in your mainline, typical of drift fishing. I've seen steelhead "takes" on side-drifted gear w/ sliding weights where all that is felt are funny twitches at your rod tip, instead of the familar weight and the ensuing headshakes of Ike. My guess is that the weight slides up the line, still tapping bottom, while the steely is mouthing your bait.
Good side drifters are awesome casters and can interpret river conditions to accurately "guess" where the holding spot is in a particular structure. Thus, another reason for the fixed weight is to present your lure and/or bait right where you want it. In deeper slots it is sometimes necessary to cast in front of the boat before making a side-drifting pass. A bait rigged w/ a sliding weight may or may not be where you want it in this situation.
A successful day side-drifting can be expensive. Nevertheless, go with your "A" game rigs. For clear to steelhead-green water I use P/P spin-n-glos, and for all other conditions, rags. Since many side-drifting spots (behind log jams adjacent to fast water, "windows" along stretches of overhanging branches, etc.) are those that boaters would'nt normally stop and work over with plugs, drift gear, etc., a proper side-drifted presentation is probably the first Ike's seen that day. So go with what's has worked for you in the past.