Name of main article should be Shimla. And redirect should be on page Simla to Shimla. As per official name of city[change source]
Dear Administrator User:Macdonald-ross, Shimla is the current actual using name of the city (You can check it by visiting the given references in article)(As the article is all about current city 'Shimla', so name should be given as 'Shimla'). Simla was just a mispronunciation by britishers who were unable to call it's right name. According to current State rules and also in the central government- the name of the city is 'Shimla' not simla.
So what you should do is create a reference from page Simla to page Shimla. Which means the main article should be name as 'Shimla'. Shimla is actual current name as per officials, while simla was just the mispronunciation of britishers. https://www.indiatoday.in/india/story/simla-shimla-renaming-shyamala-himachal-pradesh-shimla-name-change-bjp-1372288-2018-10-21
Official one- https://hpshimla.nic.in For more plz see article's references.