Both. Do both. Make it easier for them to address the issue than ignore you. Depending on which side of the aisle your MP is on, focus your letter on either “those evil <other side> are doing thus terrible thing, I know you’re bold enough to stand up to them.” or “this policy seems to have the following problems, and it’s leaving you open to attack from <other side>. It’d be a shame if you lost your position over it.”