SL97 :
Titan would be a better choice even though price/performance ratio it might not be worth the money compared to a 690, overclocking capabilities on the titan are pretty good and can easily be clocked 1100mHz which closes the performance gap between the 690 and the Titan considerably.
Also with any SLi setup your gonna have issues with micro-stuttering and some games not being optimised for it means an SLi setup doesn't always give better frame rates. Always better to go with 1 gpu. Titan is pretty much future proof for a good 4 or 5 years because when next gen cards are released the titan will drop in price and you can SLi it.
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All that you wrote makes sense, and you are quite right about some factual points. So i am just going to highlight some points about some stuff that i read till now.
1 - GTX Titan is the most powerful single GPU card on the market, at least for now. That is an obvious fact.
2 - GTX 690 is a double GPU graphic card and runs on a different level than GTX TITAN. Never the less, when compared with the TITAN is 15% (average) faster than GTX TITAN.
References:
http://www.anandtech.com/show/6774/nvidias-geforce-gtx-titan-part-2-titans-performance-unveiled
3 - IF you own a GTX 690 or an SLI of the same card, if you decide to "upgrade" to a TITAN you are actually downgrading.
4 - GTX TiTAn is 47% (average) faster than GTX 680 and 34% (average) faster when compared to an HD 7970 (all reference cards). The percentages would be less relevant on other none reference graphic cards.
References:
http://www.anandtech.com/show/6774/nvidias-geforce-gtx-titan-part-2-titans-performance-unveiled
5 - GTX Titan cost 1000$ as well as the GTX 690 and they are both premium Graphic Cards. They are designed to reach high / ultra settings when running multiple monitors.
6 - GTX Titan is a better choice compared to a GTX 690, when you have 3 monitors because of the 6 Gb Vram.
7 - You need 3 GTX Titan to beat 2 GTX Titan. That´s 3000$ vs 2000$.
8 - At 1920X1080p:
On BT Field 3, there is a gap of 27 FPS between the GTX TITAN and the GTX 690.
On Crysis 3, there is a gap of 11 FPS between the GTX TITAN and the GTX 680
9 - Micro- Stuttering issues can occur also on GTX TITAN. Take a look at this link: https://forums.geforce.com/default/topic/536434/micro-stutter-with-gtx-titan/
10 - There are rumors about the authorisation to produce TITAns with "aftermarket" cooling systems.
11 - There are rumors about a Less expensive model of the GTX TITAN.
12 - There might emerge a new dual GPU Graphic card soon. Therefore, the GTX TITAN will have it´s price droped or the new dual Gpu card will even more expensive than GTX 690.
13 - GTX Titan is very expensive, it should cost less 200$ or 300$ than it´s actuall price.
14 - GTX 690 in SLI will most probably be CPU bottlenecked when compared to a GTX TITAN in SLI.
For the record, i am still in doubght if i am buying a 690 or a TITAN.