介意 = 'mind' (v),
When you ask someone "你介唔介意 (do you mind) doing something", you are suggesting this something might inconvenience, trouble, or displease him or her.
"你介唔介意今晚同我食飯?" = "Do you mind having dinner with me tonight?"
It sounds less asserted. Implies having dinner with you is doing you a favor.
"今晚不如同我食飯" = "It is better to have dinner with me tonight"
It is not a proper sentence structure for requesting
You can use a requesting phrase like "好唔好" as in "今晚同我食飯好唔好?" to make it a proper request
Another way to make "今晚不如同我食飯" a request is to use a final particle.
Different final particle expresses different tone, emotion or meaning
"今晚不如同我食飯吖" -- 吖 (suggesting/ casual/ inviting)
"今晚不如同我食飯啦" -- 啦 (casual/ suggesting/ asserted)
There are many other ways to make an invitation. For example, '今晚一齊食飯好唔好?' or '今晚一齊食飯啦'