It is known that chlromethoxyethane forms stable carbonium ions based on Nobel prize Chemist George Olah research Olah's method. This would indicate that chloromethoxyethane reacts through Sn1 mechanism. However, Sn2 mechanism is also very feasible for highly electrophile carbon.

Which one of the mechanisms is dominant?

Organic chemistry books in this case are not very clear. For example Clayden speaks about Olah's method and neighbouring group participation(pp. 419,970 1st ed) but also gives relative rate for Sn2 mechanism(pp. 425 1st ed)