I currently have a level 22 Powertech on the Krath server and I've been seriously considering rerolling over to the Republic. I find myself in somewhat of a pickle on the issue as there are good and bad points to each side:
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1a. Republic is far more active on my server...very active and outgoing players make it a lot of fun. There are always calls for players to join in on taking down world bosses and forming flashpoint groups is a piece of cake. My Consular Shadow is in a very active guild but I will also group up with other guilds I've come to meet and know over the last week and a half. If anyone wants to know what an MMO can be then I highly recommend playing republic on Krath.
1b. Empire general chat is nearly dead. One random mis-tell every 15 minutes or so. One guild recruitment post every few hours or so. I should get a screen shot showing the same LFG post sitting in the chat window for 1+ hour. I don't know what it is this way as most Warzones means fighting other Empire players so I know they are out there. I've been kind of unlucky in trying guilds as the few I've tried have very few players (one said he was in a active guild and turned out to be the only player in that guild).
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2a. Republic stories, in my opinion, suck. I was kind of getting fed up with the game because I simply did not like the stories any of my characters were a part of.
2b. Empire has BH, and whoever wrote their story/quest arc should be getting paid big bucks at BW. Powertech also feels pretty OP for leveling as you get a healer as your first companion. I think BH should be the only class that can make enough light side choices to officially be on the Republic side if they wanted to...since they have no allegiances or anything.
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3a. Powertech tanking playstyle is just.....awesome. I love it. Part ranged, part melee, and lights everyone on fire. BW hit the nail on the head by giving us a fun class to play and a companion that is actually entertaining to have along with you (Mako).
3b. Vanguard tanking playstyle is.....unknown. Is it the same? Do you have the same kind of abilities? Also you get rDPS companion and then a squishy tank droid companion so I wonder if this makes soloing a little more difficult.
If there were paid server transfers at this point I would just ask for a good recommendation for a active Empire server and pay the money. I guess another option would be to reroll a Powertech on a more active Empire server.