You mention the word 'trustee'. You need to establish first of all whether there is a trust in existence in which case the quotes cannot be for the same work as your mother's solicitor is presumably taking into account his/her additional and ongoing responsibilities as a trustee (as well as executor).
Assuming this is not the case, if you choose to engage another solicitor to act in the administration of your mother's estate, it might be a bit awkward on a practical level to have your mother's solicitor act as joint executor. If you feel competent to act alone as an executor (with the assistance and advice of whatever solicitor you decide to engage) you might consider asking your mother's solicitor to reserve his/her right to act as a joint executor, leaving you as the only acting executor.
In any case, before making a decision, you should check that the quotes 'for the same work' do cover the same work. The cheaper quotes may reflect the work that you are expected to put in yourself as an executor, which can be time consuming. You should also check whether the cheaper quotes cover ancillary work relating to the estate administration, e.g. if it is necessary to sell your mother's house is the conveyancing fee included?