As a speaker of Hijazi dialect, both means I miss you but اشتقت لك has a friendly meaning and I usually say it when I meet the person I've missed. ( I've missed you )
But I use the other one مشتاق لك when I still miss the person and still can't see or meet her/him. Anyway it conveys strong feelings so I use it when texting ( but rarely ) . ( I still miss you )
* The last one is مشتاق لك not مشتقلك