Obviously this is very much a matter of personal preference, but I feel that the water itself may make one method better than the other.
Some rivers, like the Thompson , are wide and have large pools with a bottom of large boulders.The spring freshet carves many trenches behind these boulders and each of them can be an ideal lie for a steelhead. These lies can be almost anywhere in the river. Thus for the Thompson I believe that a lure which swings across stream is likely to be exposed to more fish than one which drifts straight downstream. Such waters I think dictate the use of drift lures.
Other rivers like the Vedder have a narrow central channel of fast water and distinct seams occur along the edges due to shorline influences. The lies in this type of water are usually along the seam which is formed near the shore. Ideally more fish will see the lure if it travels strigh downstream along the seam. Float fishing is the easiest way to make your lure or bait travel in theis seam at the proper depth.
So for this type of water I prefer float fishing.
When and how do you make the choice between float and drift.

If I'm fishing a deep hole with fast water, I'll typically go more towards drift gear, typically, I use jigs for just about everything. In my experience, when the fishing is tough, fish will hit jigs more than anything else. Several times I've gone to a river and there will be people already there drifting bait and what not with no luck. I'll use a float/jig and out fish them easily. Sometimes it's slow all the way around but it seems like jigs out produce.

It is definitely the water that dictates which method works best. As a general rule upper stretches of rivers tend to be better float/jig water while the lower stretches tend to be better water to drift fish.

Wow Coot - what a loaded Q! The suggestions could take a lot longer than will fit in a post; but I'll give it a short shot. ...

In general, float fishing is the obvious preferrence in real snaggy holding water. As for the big boulder holders in large rivers such as BC's Thompson and Oregon's Deschutes rivers (or not so big of rivers for that matter), sometimes you can keep your bait or lure right in the slowed water behind them for a bit before they work on out, down, and swing. Some of these spots actually have a current reversal, and the fish don't lay in the reversal facing backwards as often as they lay just on the seam between the opposing currents that are created by river/boulder hydraulics; or back a ways in downstream currents that still have a slowed affect from the big boulder up river. If I can use a float, and keep my mainline up off the water with a long rod, I like to let bait or jigs or pink worms hover and wiggle around in the swirling area behind the big boulder where fish close by laying along the seams easily see these offerings. Some will go right after them, while others will get aggrivated into striking at them after a bit if they become annoyed with the offering. You can also drift fish this hold by casting up and beyond the boulder and allow your rig to drift into this same hold - the trick is to keep it in there for a bit before it drifts on out, down, and then on into a swing over. Long rods held high, along with thin diameter lines, aid with this type of presentation. Using a little heavier 'reduced snag' slinky weights help also. ...

When drift fishing slots or runs without boulder or log 'holds', there has been a lot of recommendations by accomplished anglers to use just enough weight to tick every 5' to 10'. This type of fast presentation does work well for steelhead, and is a good thing to do at first because steelhead respond well to fast presentations; and if the speed differential between surface and bottom currents isn't much, then it also looks more natural to them. But if that doesn't produce, I suggest following it with added weight to slow the speed of the rig's drift for the less aggressive steelhead that may take a slower presentation instead. This is particularly the case in either murky water, where the fish will have more time to see the offering, or very cold water when the fish are more lethargic and often won't move as far or as fast to grab your drifting rig. But remember that the water along the bottom, where the fish most commonly are, is often moving at a slower speed than the surface water. Because the surface water pulls your mainline and/or float, the presented bait will be faster than the water along the bottom where the fish are holding. This is another reason to follow the faster light lead drift with a slower heavier lead drift - because it can actually look more natural to the fish in many cases, despite the movements caused by lead hitting bottom. It's a little harder to slow the surface speed of a float though. That's why it's usually best not to use them in the fastest parts of a hole; with some exceptions for shallow water summer steelhead presentations. Use them off to the side in the seams, the slower lower half of the hole, and in the tailouts. Otherwise you may need to be above the float and slow it slightly with a little bit of smoothly applied line tension. Also remember to switch colors or lures if you know fish are there but haven't taken what you served up yet.

It is also good so slow your presentation for chinooks in the river. They often prefer a slower drifting bait. You can do this by adding weight, as mentioned. Or by just fishing the slower deep parts of holes or along seams where your drift fished or float fished offering will naturally be slower, and where the 'nooks hold more often anyway.

What was the question? I forgot. Just kidding.