Here is a comment I found on this website * that is worth sharing... * http://space.stackexchange.com/questions/3161/why-is-spacex-considering-methane-as-fuel-for-their-next-engine-the-raptor
"Methane (CH4) and RP-1 are roughly equivalent in realizable performance. As previously mentioned by other posters, CH4 has slightly higher impulse – about 370 s in vacuum vs the 360 s – at the same chamber pressure of 7 MPa. But, this is counterbalanced by its lower bulk density of about 830 kg/m3 vs about 1030 kg/m3. Bulk Density is the density of the combined Fuel and Oxidizer load in their appropriate ratios. Even though Methane is "only" 430 kg/m3 it is burned with 3.5 parts oxygen compared to 2.1 parts for RP-1, hence a CH4 rocket will be carrying more oxygen and less fuel by weight. Oxygen is pretty dense at a little over 1140 kg/m3 which is denser in fact than RP-1 (about 810 kg/m3). If we assume that chamber pressures and engine cycle efficiency will be equal, RP-1 outperforms CH4 simply because a 20% larger tank will impose weigh penalties that slightly outweigh the 3% increase in specific impulse. However, the RP-1 advantage is contingent upon operating at an equal chamber pressure which may not be the case. And, Methane (CH4) has additional advantages that are applicable in specific scenarios.
The reasons CH4 is a front runner for SpaceX's Raptor can probably be attributed to four factors:
1) Methane does not coke (polymerize) at the operating temperatures of a rocket engine – it's coking point is roughly twice as high. This makes it easier to make an engine reusable and re-usability is a key SpaceX objective.
2) Because Methane does not coke, it is also easier to implement a full-flow stage combustion (FFSC) cycle where all the fuel and oxidizer flow goes through the pre-burner. Compared to contemporary Russian partial flow stage combustion engines higher chamber pressures are attainable resulting in a total impulse advantage of about 30 seconds, or 9%. This eliminates the performance deficiency of CH4 compared to RP-1.
3) If SpaceX intends to use the same fuel in all the stages, CH4 can be considered a better upper stage fuel and a worse lift-off fuel, even without enabling higher working pressures. This is because upper stages are typically 1/8th to 1/10th the size of the 1st stage, and here impulse is more important than density. Using Methane with the aforementioned FFSC cycle means that SpaceX can potentially get equivalent 1st stage performance and better upper stage performance.
4) Even though it is, IMHO, somewhat dubious that early Mars mission will use in-site fuel production. If that ever becomes an applicable practice, Methane can be produced from water and CO2 while RP-1 cannot.
Other than that, there is the non-factor that somewhat favor Methane, such as regular grade Natural Gas being good enough and not having to highly refine the fuel from regular kerosene to RP-1 to achieve low coking characteristics and consistent densities. I say it is a non-factor, because fuel cost is such a negligible part of launch costs that it really doesn't matter if fuel cost a few times more or less. Fuel is typically only about 0.3% of the cost of flying a rocket to orbit, so fuel cost really doesn't matter – Not even when you compare highly expensive propellant combos like Hydrazine/Tetroxide to the relatively cheap Kerosene/Oxygen."
On #3.... He is somewhat wrong when he claims that Methane is a "worse take-off fuel";
A) What he said in #2.
B) Methane gels and slush should be far superior to all Kerosene based equivalents. - Approaching LH2/LOX!*
C) Methane (in general) has an isp of roughly ~10-20 at sea level. For example, one of the most efficient RP-1/LOX engines at sea level is the RD-180, at ~311 isp. The RD-192 engine, which runs on Methane, will have a SL-isp of around 330! That may not seem like much of a difference, but it could add another ton to orbit. - That also is only ~30 isp away from the SSME, not bad at all!
D) Even though Methane has a lower density than RP-1, it is still high enough to have the "violent launch" that Hydrogen doesn't have... (The increased density of RP-1 doesn't mean much as improved turbopumps on Methane-based engines could make up for the loss.)
* Slush Methane won't actually increase the density of Methane very much, but it will allow for the addition of aluminum particles etc to increase isp. It is also easily achievable, unlike the much talked-up slush-Hydrogen.