The Shadowsword kit comes with six possible variations (datasheets on the GW site here). Each has various sponson options, mostly lascannons and heavy bolters, but a few have some specialist kit (noted ineach entry). I'm not discussing points as there is very little difference with upgrades, at least at an Apocalypse level. Here are my thoughts on them individually (in alphabetical order):
This beasty is one of the two that can carry 25 dudes (ten of whom can fire) but still has a nice big gun. The gun on the Banehammer has a good 60" range, Str8, AP3 and a 7" blast, so good for annihilating enemy infantry. It is also Ordnance so can be used against armour with some reliability.
It also features the nifty earthshock ability, where a 4D6 "size" (it isn't clear on diameter or radius, but most measurements seem to be diameter) area becomes difficult terrain, with one less dice for the test. It is also dangerous terrain for vehicles.
Quite useful as a mid line support vehicle - use the cannon to tie up enemy units and load the boot with heavy weapons or other support assets.
The artillery version of the Shadowsword, the Banesword features the Quake Cannon as its primary gun. This beast fires a barrage 24-180" away. That's a Str9 AP 3 10" ordnance barrage. It's a 10" blast so even the guard can hit something, and if you feel insane you can still fire at things directly under 24".
Depending on the table layout you use the Banesword can be very, very good or very, very, mediocre. If it is a nice big table and you have some cover you can park this sucker behind some nice big cover and blow up scads of infantry and light vehicles with it - you're still hitting with a Str5 hit with the main blast, and get to re-roll the armour penetration dice don't forget.
If it's a long thin table or not much cover the long minimum range will hurt, as the main benefit of the Quake Cannon is its indirect fire ability.
Rating: 8/10 or 6/10 depending on the table.
The Doomhammer instantly wins point for being able to bring dooooom to your foes. Also, it has liquid hot mag-ma.
The other 25 body transport, the Doomhammer is a hybrid Shadowsword/minibus. The Magma Cannon fires 60", with Str10, AP1 and a 5" (large) blast. Again it is Ordnance, so the primary duty here is anti-armour - Land Raiders, other SHs, GCs etc.
However it bears the Imperial Guard's wonderful BS of 3. With the smaller blast it will be harder to hit a target at all let alone with the centre, unless you're firing at something big.
The Doomhammer has a few good parts, but suffers for trying to do too much without doing anything particularly well. Which is a shame as it has the greatest comedy potential.
The grand-daddy of them all, the original superheavy that isn't the Baneblade, the Shadowsword exists purely to kill armour, and big armour at that.
The Shadowsword's Volcano Cannon is the smallest of the guns to bear that name. It has a massive range (120"), but that generally doen't come into play. Its main drawcard is StrD - the only pure Guard vehicles to have it I believe. Compared to titan VCs, the Shadowsword has the smallest blast at 5" (large blast). Coupled with the IGs middling BS, it is a bit more risky than the Titan mounted weapon systems to score a hit.
The Shadowsword does have the advantage that it comes with integral void-shield busting lascannons, but the BS3 firing them strikes again. Basically only take a Shadowsword if you know there will be other SH vehicles to kill, otherwise there are things that kill tanks better for cheaper.
Rating: 8/10 or 4/10 if they don't have superheavies.
"It has All Power to the Weapons!! It has 30 shots!!!"
But, the cynic replies, it has Str6 and BS3. I have ranted against the VMB before, and this does not change when it fires more with a lower BS. The damage output is lowered by the BS to counter the high rate of fire so there is no significant change in kills.
Also, when firing 30 times you can't move. So the mighty assault vehicle can't move anywhere, and even if it does it only moves 6" a turn. You can assault out of it, but it's not likely to get anywhere anytime soon.
There is a popular idea of cramming 20 or so heavy weapons teams in the back. This often falls down as few people have 20 weapons teams, or if they do they need them for their squads. If you want to do something that stupid you'd better have the models to back it up.
Suffers similarly to the Doomhammer - an assault vehicle that moves slower than infantry (no run move), and a massive gun that's not very good at killing things. The best use anyone can come up with is a mobile pillbox. When the best use for a superheavy is as a bunker for other things that can actually kill things, it's time to go back to the drawing board.
Rating: 3/10 - yes, I have actually used one in a game. It's great as a tank or movable building, but as a superheavy it fails miserably.
One of the oldest variants on the Shadowsword hull, the Stormsword carries no troops, just a giant assault gun.
The gun has a short range (36"), but a massive Str10, 10" ordnance blast balances that out. It also ignores cover so you can kill anything nearby with impunity. Good against all infantry and a large number of vehicles, the cannon only suffers that you need to get in close.
In that regard it is the inverse of the Banesword - if you play on massive boards you will need to flank march to get the best out of it, but on a close in board it will murder your opponents.
Downside is that the 10" blast doesn't care who it hits, so friendly infantry need to be wary of standing too close.
The default sponson loadout is a bit odd, with heavy flamers in place of lascannons, due to the Forge World version coming with them by default. You have to pay points to upgrage to lascannons, and to upgrade the hull heavy bolter to twin linked. However I'm not so sure that's a necessity (maybe the hull bolters) as the Stormsword is designed to be in the thick of it killing the bejeezus out of the enemy - the goal of any superheavy.
Rating: 9/10, or 4/10 on a sparse map.