Auto ignition method gives better result since citric acid acts as a fuel in the combustion reaction and crystalline nature of the powder increases.

In combustion method, at higher temperature, crystalline structure improves.

while citric acid in both types acts as a surfactant.. 

According to my knowledge in combustion method your precursor should be nitrate based where NO3- is the oxidizing agent and citric acid is the reducing agent. Here, auto ignition occurs with the decomposition of fuel. In sol-gel u don't have the restriction on the type of precursor.  Here, just decomposition of the chelating agent occurs. Combustion reaction prepared material shows good crystallinity, small crystalite size and high porosity.

It's hard to tell which product is better than another without considering your application of these powders you can obtain from these two processes. In my opinion, the key factor here is the calcine (or auto-ignition) temperature, since higher one always leads to better crystallized but coarser powders. If you want to use the high specific surface of the fine powder as the driving force of latter sintering, then you have to always try to maintain the calcine or auto-ignition temperature as low as possible.

Ok, what is the conceptual difference between these two methods, as same citric acid is used in both cases. The sol gel synthesized sample is heat treated (calcined). Won't there be combustion process during calcination. Are both methods same?

yes, combustion prevails in both the cases.  since citrate is a source of carbon which on heating consumes oxygen,and very well you know  the reaction as it happens  in burning of carbon based compounds, the difference is that achieving the combustion temperature, with a maximum exothermic energy which distinguish the process in sol gel citrate method. the auto combustion process usually involves the exact stiochiometry of oxidizer to reducing agent(O/R), which provides high exothermic energy or flame temperature to complete the reaction without any residue of carbon. essentially the Ph also play a vital role in achieving the lower combustion temperature for citrate combustion by adding ammonia,  were the combustion temperature appears to be very close to >150 -200 oC in the stiochiometry (O/R) which gives enormous amount of heat energy.  ( if no Ph addition is maintained) then smoldering combustion happens in that temperature, increase the temperature of the gel leads  to burning of citrate with less exothermic energy happens. Also the chelating nature of citrate wont be effective in the later case.