'If you choose to work with us... '
'If we get the opportunity to serve you...'
Two lines I read from reading through two detailed proposals.
Without any exaggeration this type of language should not be underestimated in its implicit weakness.
It may sound reasonable but all it does is give the power to your prospective client, putting you on the back foot.
'When we begin our partnership...'
'When we start working together...'
This assumptive use of language consisting of a presupposition (that you will start working together) is powerful and demonstrates confidence.
It clearly shows that you view yourself as an equal partner, working together collaboratively, without giving any power away.
There is no pandering, no desperation. It clearly communicates your level of conviction.
If this is your approach and it comes through in all aspects of your written and verbal communication, it could well be the difference that makes the difference.