If your facility is subject to state boiler inspection, you may not have a choice.
ASME approval is not required on water heaters with input of less than 200,000 BTUH and tank capacities under 120 gal.
Above those capacities, most states require ASME …including expansion tanks.
As far as I know, the exceptions are:
Alabama
Connecticut
Kansas
Montana
North Dakota
South Carolina
South Dakota
Wyoming
Note that state and local codes are subject to change, so, the best answer is to check with your local jurisdiction.