I've done it both ways and now prefer slicing before freezing. As others have said, they key is to leave the juices in the sliced meat, freeze it so the juice doesn't come out during the vacuum process, and then vacuum.
I never noticed much (if any) difference in taste, and that is the only potential advantage of not slicing before freezing. But while not slicing doesn't buy you much, there are quite a few advantages of slicing before you vacuum pack:
1. You only have to clean up the slicer one time. Even though my Chef's Choice 615 is a simple slicer, it still takes quite awhile to clean. If I have to haul it out for each leftoever meal, that is a LOT of extra work. This takes much of the fun out of doing leftovers because, as I describe in #2 below, pre-sliced leftover meals are about as simple a meal as you can get.
2. Your leftover meals become ridiculously simple: just heat and serve. I stick the Foodsaver vacuum pack containing the leftovers into some hot water on my induction cooker. The cooker has a temperature regulator. For meat that should be kept rare, I set the induction cooker to 130-140. For meat that was pulled, I heat the water hotter. Those of you with sous vide, you should use that for reheating. I have a homemade sous vide made from my crock pot that takes a few minutes to set up, so my induction cooktop is easier to set up. The temp for reheating is usually not very critical.
3. While preparing each vacuum bag, it is MUCH easier to divide up the meat evenly if you slice it first. While I certainly do not obsess over portion control, it is nice to divide the leftovers into something that is roughly what you want to consume. Everyone is different, but my wife and I each eat about the same amount (actually she eats a LOT more than I do), and I have found that five ounces of cooked meat makes a very good dinner portion for each of us. So, what I do is take the entire leftover amount, and stick it on a scale. I then take whatever reading I get, and do the math to find out what number of portions will give me something close to ten ounces per bag, or a little more.
Here's the problem: if the meat is not sliced, I have to guess, just by looking at the slab of meat, where to cut. Unfortunately, the meat is usually tapered and therefore not easy to cut equally just by looking at it. I do my best, make my cut, stick it on the scale, but then either have to trim a little or add a little from the main hunk of meat. I get close, but it takes time. However, if I first slice it, I just keep adding slices until I get to my five ounce (or whatever the math tells me I need to weigh) target. Simple. Easy. Exact. Fast.
