No... I've held a cut piece of tubing and the diffs for a given length of high end steel tubing is not double the weight of the appropriate length carbon...
Remember that an ISP doesn't have a big overlap of materials like a standard seat post that gets pushed into a frame will have. And most carbon seat posts are built with fairly thick walls to fight being crushed.
So an ISP will be the weight of the tube length and the mast topper. Fairwheel have mast toppers at and below 100 grams. The steel seat tube for the ISP will depend a lot on the geometry, but a full length tube that is 25 inches of ms2 straight gauge steel tubing (28.6mm) weighs 340 grams. That's only 13.4 grams per inch and most folks will only have what, 5-6-7 inches? 80 grams for a 6 inch ISP... Thats an ISP weight some place near 180 grams plus or minus...
For a traditional seat post you'll have the seat post it's self, including the saddle mounting hardware and that will likely be near 200 grams. You'll also have the seat post clamp and bolt that for steel is going to weigh 10-20 grams...
This is just generally speaking and people can tune both things, but I would bet the seat post versus ISP difference would be damn near nothing for most folks. Certainly not a couple of hundred grams.
I had this same debate with myself. I have a custom frame built from Columbus Spirit and my builder thought it would be better to just cut a seatpost short given my other desires: I wanted a pretty traditional style "parallel to the ground" top tube and the ability to adjust up and down a few MM for different saddles, etc. You can do this with a mast, but a Thomson cut down for the same frame is pretty light (I'll have to weigh it... but FWIW Im using an Elite at the moment cut with just enough in to be safe, debating on setback or 0 setback Masterpiece). It's half an aesthetic thing, which makes it hard.
If in doubt, go with a seatpost. If your frame is going to be more compact (and you need more post as a result) then a mast will probably be lighter.
As for carbon vs ti vs alu... it depends on your cockpit, I guess. With steel I think you don't need as much deflection (though I am debating a setback post) and Thomson (and a few others) make alu posts that are just as light (or lighter). I ended up with a Thomson stem and using the old rule of stem matches bars made it a no-brainer. If you're hellbent on the lightest setup, ignore that rule.
If you get a mast, see if your builder will do an integrated stem/bar... that would be very pretty and rather light.