Go with the mobile marines kit, there are very good models and easy to construct for the beginner and expert alike ( with optional parts if you want more then a basic kit) , Mountfleet do good kits but it would be a fair comment to say they are for the more experienced modeller, and a lot more work to expand off its basis.
In regards to scale if the real tugs are different sizes it would be that the models are too hence the length difference. But one model may be inaccurate i have no way of telling though idle speculation.
Either way the choice is yours!