It would depend on what BOFF layout you would have. The Excelsior requires 1 Tac, 3 Eng, and 1 Sci, while the Star Cruiser requires 1 Tac, 2 Eng, and 2 Sci.
Assuming you are going to adjust your BOFF setup to match, or have the Officers to fill either ship, the Star Cruiser comes out better for Tanking due to its additional Ens Science slot and extra Sci console. This translates to more Shielding (with a Field Generator console) and an additional Ensign Science ability such as Polarize Hull.
The Excelsior is designed to be a more balanced design. It can tank very well, but it is meant to have more offense than the Star Cruiser at the cost of a loss in maximum tanking potential, even when compared to ships like the Galaxy-R. This is the same tradeoff the Assault Cruiser makes, and it works for both ships for their roles. However, if you are only interested in tanking ability, I would say the Star Cruiser comes out the winner.
To be honest with you if you are looking to tank the best choices would be an odyssey or a star cruiser as the excelsior is bent toward the term "Assault" Cruiser and thus has more to offer in the offensive role.
The Excel is an odd ship with an (AFAIK) unique layout. For a PvP cross-healer there are better choices (and other players would be far more qualified to help you there), but it can do an admirable job nonetheless; its main drawback is the average-at-best Sci setup. Basically the Excel's Lt. Com Tactical slot is a wildcard that drastically changes the way the ship plays.
I have not experimented with this yet, but one thing to consider is that thanks to Technician (?) DOFFs, people only need 1 Engineering Team now instead of two. It could very well be that this makes the loss of one Lt.Cmdr Engineering slot irrelevant. You can still fit 2 Emergency Powers, 1 Engineering Team, an Aux2SIF and an Extend Shields this way, though you will have to take a somewhat lower level version of one of these skills.
Star cruiser tank here. In my experience, the star cruiser is an amazingly durable ship, but really wilts in the damage/aggro area. with 9 ranks in threat control and FAW2, I sometimes still cannot hold aggro. And if I'm not holding aggro, then only 1/2 of my abilities 'work.' In case you're curious, here's my star cruiser build:
7 AP 1 torp
2 borg 2 MACO mk12
tac LT: tac team, FAW2
eng commander: EPTS1, RSP1, Extend Shields, Aux2SIF
Well, the Vanguard has better survivability, but the Excelsior trumps it in threat generation.
Secret option 3: if you've got 1600 c-points to use on an Excelsior, you could splurge 400 more and get one of the 3 Odysseys.
Survivability-wise, it's probably Odyssey Ops>Odyssey Science>Odyssey Tac, threat gen-wise it's probably Odyssey Science>Odyssey Tac>Odyssey Ops; making the Odyssey Science the best of the 3 overall, but all 3 can be excellent tanks.
Note that comparing the Star Cruiser to the Excelsior isn't altogether a good idea. One is a dilithium ship you can get for free (via leveling) that is built to have a Science emphasis, and thus a very good ability to tank, while the other is a C-Store ship you have to pay real money (or huge amounts of dilithium) to get that has a general emphasis, and thus a better ability to do damage at the expense of a lower tanking ability. Unless you have another C-Store ship you might be considering, if you have the resources to get either ship, you will probably end up buying both, so a decision isn't as necessary.
Comparing it to the Assault Cruiser might have been better, as both are dilithium ships that you might have to choose between, and the Assault Cruiser performs very much like an Excelsior (minus the Transwarp Drive ).
To sum up...if you don't have the C-Points for an Excelsior, you don't really have a choice between the two. If you do have the C-Points and want the Excelsior, then you can get both. If you have the C-Points, but don't want to get the Excelsior, then you can choose between the Assault Cruiser and Star Cruiser.
Last edited by danqueller; 06-25-2012 at 10:18 AM.
For pure tanking, note that while a Star Cruiser can do paired EPTS3, an Excelsior can't. Plus with the extra ensign slot being science instead of engineering, and at the ensign level there is a lot more you can do with an extra sci than an extra eng.