Depending on the type and brand of engine/generator, some brands of controllers are more accommodating than others. In my experience, with GE gas turbines (GE10, 7FA, etc.) using GE controllers was the easier obvious choice. For a Kawasaki gas turbine, ABB worked well and was more cost effective.
Some reference links below -
http://www.geautomation.com/products/mark-vie
http://new.abb.com/control-systems

10 MW is a small machine and it will be economical to stay with a control system provided by the machine manufacturer as standard. It will be costly and time consuming to custom-make and program a control system using PLC/DCS from other manufacturers. Coordination of the signal inputs and control outputs with the machine manufacturer is the hardest part.

I am familiar with Rockwell / Allen Bradley but also was told that Rockwell is harder to program than others (such as Omron, Siemens, etc). Personally I like simple straightforward applications and not many windows and parameters to work with but this might be secondary, depending on what you are looking to accomplish. Also one would think protection & control relays can be more important than PLC/DCS when it comes to generators. Have you identified other components, ie relays, etc? If there is a staple product, the design might be established around it to ensure compatibility.

You've received good answers here. I'll add that PLC will be more cost-effective. If you have the generator supplier include the PLC, hardware and software you'll get the best functional product at lowest risk providing you or your client do not want much involvement in the design. They will have a canned repetetive consistent product. If you have a client with desire and saavy to be involved during or after implementation with some reason for customization, use a control system integrator.
I wouldn't be afraid of any of big name DCSor PLC suppliers. Best is what is big in your market so you get delivery and service.
DCS gives a more integrated solution than distinct PLC and HMIsuppliers. Not a problem though for an experienced integrator.
If this is a brownfield installation, use what DCS or PLC they plan to be using for next 5 years.
BTW, this interface is frustrating. Wow

This is very application dependant.
Diesel (Fuel Oil) engines can get away with very low levels of reliability and generic hardware.
Gas fired engines & turbines will require minimum SIL 2 certified controllers in most first world jurisdictions like North America, Europe and Australia. Similar requirements apply to steam boilers and turbines.
If you are not using vendor hardware, Schneider Triconex does third party packages based on Triconex Tricon (SIL 3), Trident (SIL 3) and TriGP (SIL2) and Siemens has similar setups based on the S7F series PLC’s.
Most offshore and marine applications will require additional certification requirements including specific cabinets to be used.
The advantage of the Triconex hardware is that it is fault tolerant and module replacement online is possible without shutting down the engine or load (useful for marine main engine applications). It is not cheap however with a typical PLC installation costing over $100,000. (The oil & gas industry love them for this ablity - i.e. zero downtime due to controller faults that will kill other SIL rated systems).
A lot of SIL rated vendor systems are fail safe (hard trip on fault) not fault tolerant (run safely with fault) and the requirements for this need close investigation. If you can live with annual spurious trips then most SIL rated PLC's will work (MTBF spurious/safe of 8 years per card is typical) otherwise a little more design work is required.

Interesting Discussion Really and amazing responses. I might not have presented my question very well. More detail about the Engines (10MW Diesel Engines of Sulzer brand). Machines have been bought from scrap but with condition of engine with no control system provided so verndor is looking to design a control system for Engine and generator which will be called Engine and generator control systems as architecture.
Vendor is looking to use PLC as logic controller to design engine and generator control system so which brand will be most suitable to use ?

Thanks philip, Dilip, Inci, Les and Allan for such great contribution of your time.

Khalil, you posted a very good question, an interesting subject. I would think, protection relays & IEDs are more application specific than the PLCs, given as Allan pointed, the PLCs for this application should be SIL3 for redundancy, such as Modicon or others. On the other hand, the PLC is just the controller, one would imagine it needs to be programmed, tested, equipped with protection relays (generator), brekers, etc. and done redundantly.
As Philip pointed out, there are ready to use fully equipped genset controllers, personally I would go with one of these, to avoid reinventing the wheel, unless the price difference is huge. You can shop around but personally I would check with ABB first, as theirs seem to be straightforward and simple.
Out of curiosity, are you able to forward electrical drawings for the generator?

Great, Amazing answer really @Inci. My client is doing feasibility report but have not got drawing yet, sorry. Yes they are planning to use various relays, Generator protection, Engine protection and differential protection. Which brand do you recommed for these relays ?
I do not know the price difference yet TBH but I should check that as well.

Personally I think any brand that has generator & engine protection relay line will do, contact vendors and they will give you a proposal, then you can compare the offersto make a decision.