Hi @JyoshnaRani, thanks for reaching out. Either of the two SDKs can target iOS devices but ArcGIS Maps SDK for Swift replaces ArcGIS Runtime SDK for iOS. The iOS SDK is under long-term support and will only receive bug fixes and minor updates moving forward. The Swift SDK on the other hand is under active development and users can expect regular updates. Developers using the iOS SDK could write apps in either Objective-C or Swift. On the other hand, the Swift SDK is to be used solely with Swift and makes use of the language's latest features.
Overall, for new applications, ArcGIS Maps SDK for Swift is the recommended choice.