Pulling some numbers out of my rear, so take it with a grain of salt if you can stomach them...
Going with the figure of .081lbs per CF of air (don't remember where I got that number from), double 120s at 240cf of air would weigh 19.44lbs. Looking at PST HP 120's, they are 36 lbs each, but have a bouyency of (-0.4) lbs, bringing you up to 91.44 lbs out of water, but only (-20.24lbs) bouyency. Toss in about 10 more pounds for regs/manifold/hoses, bring it up to a little over 100lbs on dry land, (-30lbs) bouyency. Then add in any other gear that you want to tote around (BP, knives, extra masks, compass gagues, dive comp, lights, etc) bring dry weight up to say 120lbs and the bouyency up to (-45lbs).
A person is usually a few lbs bouyent (which is why people tend to float), you can credit yourself a few lbs, I'll go with 2lbs for a nice round number.
If you are 200lbs on dry land, add in the 120lbs of gear and multiply by the 0.025 for increased saltwater density displaced (not very good meathod, but acceptable for eyeballing bouyency to get an estimate) if diving in salt water bringing you to anoter free 8lbs bouyency.
Not being a drysuit diver myself, I have no idea how many lbs of bouyency you can get with your drysuit with normal operation, but I'd wager it's at least 10 lbs (and this 10lbs bouyency can be maintained at varying depth unlike wetsuits).
This would make your initial bouyency at entery without weights (-15) and your total land weight that you have to drag off of and back onto the boat at 320lbs.
If you suck your tanks 3/4 dry (easy fraction to compute with), then you would have a bouyency at decompression stop of ~ 0lbs, dry weight of ~305lbs.
Looking at LP steel 104's in comaprison, they weigh 45lbs each with a bouyency of (-0.7) and the air would weigh
~16.8lbs for a total of ~106.8lbs dry, bouyency weight of ~(-15.4)
Assuming the rest of the gear is the same and your weight is the same, in salt water, you will be approx 5.4lbs more bouyent at start and 4.4lbs more bouyent at 1/4 air left over the HP 120's and approx 15lbs heavier out of water.
edit - Since that make you 4.4 lbs bouyent at the end of the dive, you would probably slap on a 5 lbs weight to offset that to maintain neutral bouyency at deco stop, removing the extra bouyency at the start of the dive as well, and making you 20 lbs heavier out of water.
Diving steel doubles with a wetsuit is not generally reccomended for bouyency at depth safty reasons as outlined by many others in this thread, though I don't doubt that it can and is done safely by some, be it by bringing lift bags or what have you.
Edit - I vaguely remeber reading on some site that was a pretty strong advocate of DIR that they advise against it for wetsuit open water dives, but I can't for the life of me remeber where I read it, but will try to locate it later.
The reason to get steel HP120s over steel LP104's should be because you need the extra 36CF of air and have weight issues out of the water or size issues that do not permit steel LP120's.
The reason to get steel LP120's over steel HP120 is because while heavier out of water and larger, in water they have simlar bouyency characteristics, carry the same CF of gas and are easier to fill with regular air (some smaller compressors in the boonies/ on boats/etc may not be able to fill HP tanks to + rating as readily as they can LP tanks) or increased difficulties filling HP tanks with nitrox/trimix mixes.
