I guess what really bothers me about threads that I read like this is that posters seem to adopt a sky is falling stance when trying to explain their point.
I also really hate all the negativity. It would be nice if for once a rationale discussion could be had without comments like "facerolling, clueless, etc etc."
For the record I am a Shadow tank. I have had considerable success with my role in my own experiences. I wont presume to claim that my experiences match those of any one else's.
For the OP: Please choose a concept that appeals to you the most. I would interject here one thing however that I believe bears some relevance to your thought process. In your post you stated that you would like to go as a DPS every now and again within your advanced class. That's fine and certainly doable but there is a particularly annoying mechanic right now that does impede that slightly, and that is not having the ability to switch specs without paying for it at your fleet or capitol city. Also doing so on a regular basis can become rather expensive. I'll admit that like you I would like to have the option without the costs becoming prohibitive, but that's how things are right now. I point it out solely because I think it allows you to make a more informed decision on your final choice of class.
As for the whole Guard/Shadow/trooper and Sith equiv topic I really only have one or two things to say.
1. While my personal tanking experiences have been mostly limited to the shadow I have observed on many occasions the other tank types doing just fine against pretty much everything the game has to offer right now. Will certain classes have an easier time under certain conditions and in certain scenario's? Possibly. I'd even go so far as to say more than likely yes. However, as long as one particular class isn't the preferred one over and over again in those scenario's that is ok. All that really matters is this - Can I as class XXX do my job? if the answer is yes then there's really no problem. The amount of personal effort required to accomplish that goal really isn't an issue because for the most part what is just right and what is too much is going to be very different between individual players. I.e some people might think the extra required effort is more fun. Some people wont. That's just the way things are. At the end of the day if you can accomplish your goal that is real priority.
2. As a Shadow the only time I really have a hard time tanking is when I find myself in a situation (either solo or in a group/op) where I have to tank multiple npc's at once. This isn't so much a threat issue per se, as I can single out what needs to be tanked and what can be left as fodder for the dps, but rather dealing with the mechanics required and the incoming damage. I've found that say as a off the wall hypothetical that 10 little guys/strongs are far far more deadly than 1-3 elites/champs. Obviously this is where CC must come into the equation and how much of a threat they are depends overwhelmingly on what particular abilities the NPC's have at their disposal but I am constantly on the lookout for more effective ways that I can personally implement that will make these situations somewhat easier when they do occur. So, if anyone has any advise to give in this regard I'll gladly take it even if it's something I might already know. Hopefully somewhere I nice little gem of knowledge will surface that I can take advantage of.
3. Lastly try to adapt your play to what works and MOST IMPORTANTLY what makes the game fun for you. Changing game mechanics are likely to be slow in coming and very small when they do come if they come at all. It's not a quick process to re evaluate things and re develop game code even if it seems clear cut to you as an individual. In the grand scheme your one of many hundreds of thousands of people each with different opinions that developers need to try to make happy. And despite what you might think chances are really good that a specific issue that you think everyone else is up in arms about really isn't that big of a deal to the vast majority of the playerbase. I'm sorry if that upsets anyone but that is reality. I encourage people to post their concerns in a constructive and polite manner, but a far more effective method of playing is likely to be to adjust your own expectations and playstyle to the game that is rather than the game you might want it to be. I hope that made sense.
I guess that was 3 instead of 1 or 2 sorry. Sometimes I tend to ramble but my intentions are good I promise.
See you folks in game.