I've played the Total War games - including the lastest Shogun version - and Civ games since the first ones came out.
Other than being turn based games, the games are very different.
Both share a few things in common. Whether called science, art or technology, you advance through a tech tree to obtain advantages on your path to victory. You also develop cities (or castles) along per determined paths based on the opening up that tree - as you advance further, you can build newer buildings or new type units. You then use these building and unit types to try and expand your empire.
In Civ 5, you usually have the ability to do this in a variety of ways, and have a new map every game. In Shogun, its always the same map - Japan - and cities (castles) are never destroyed completely. The warring factions don't start with the same resources, starting forces or equal strategic positions or advantages. You change factions for a "totally different experience". Which are much different that the different civs in Civ 5 - the differences in Civ 5 are relatively minor compared to these starting differences.
But unlike Civ 5, Shogun centers the game on warfare and you will win or lose depending your skill on the battlefield as well as how you can supply your army.
Shogun has two games. The grand strategy game allows you build up your castles, deploy your armies, and do diplomacy. Its is turn based.
The tactical game literally has you fight each battle on a tactical battlefield with what every troops you have at that battle. Archers, melee units and cavalry all have very different roles - and the battle done in real time, not turn based. The results of this battle are then translated to the grand strategy map, and you live with the results.
Civ 5 takes on a more grand strategy let's look at history from dawn to the end of the space race approach. Its a race game - either to science, culture, domination, etc.. - centered on getting to the victory conditions first. And you have multiple paths to victory centered around building an civilization that will include war but is not, necessary, centered on war. You can win in a lot of different ways.
Shogun's highly focused on Japan. You see a historical period in great detail and are immersed in that historical period. Time, unlike Civ 5, stands still. Its Medieval Japan. The additional technologies, units and positions make it more Japanese and you don't see new historical era's come into existence.
I find that Shogun gives you a totally different experience than Civ 5. And I don't mean better - as I enjoy both games, and both games have bugs. And we can sit here and discuss shortcoming for hours.
But the experience is different. And I haven't had any replay problems with Shogun since its come out.
But if your new to the Total War Series, you have to give it some time to get used to it. The game - like Civ 5 - not very well documented AND you have two games to learn - the strategy game and the tactical game.
As far as forums go, both Civ and the Total War series have good and bad forums. You will have to find ones that fit your particular taste. I like the
http://www.twcenter.net/forums/index.php.
The people are usually polite and helpful, its realtively well moderated, and has some good advice on the game.
These two series are the only ones I've "followed" this long. Both offer, in my opinion, a good game and a lot of enjoyment.