The Original Novels both of these stories are based on were written by Ira Ishida. There are a lot of similarities in terms of atmosphere and setting, by the method of which the districts of Ikebukuro and Akihabara are given unique personalities. Both of these stories are urban mysteries where the protagonists investigate various crimes occurring within the city.
Ichi the Killer is a much more intense and doesn't come with the comforting "good guys" redeeming element that Ikebukuru West Gate Park does. However they're both very eccentric stories that feature gang violence and revolve around trying to understand who and why someone was murdered with plot twists related to mental health. They also both feature manipulative antagonists and homoerotic tension. Be warned: Ichi the Killer is fucked up.