You should always select a company that is reputable, and committed to remain in business. The cheapest contractor will usually not stand behind their work because in most circumstances this cheapest contractor will sub contract out the work and the sub they used will not be there to fix your issues should they arise or an additional fee will be presented to fix an issue that should have been covered in the original contract.

Clarify the warranties, get them in writing, select a landscape designer that listens to your vision and needs. Dealing with a company that designs and builds is usually a safer way to achieve your goal.