New Bike or change the old one?

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My old bike isn't very old but it is a MOTIV mountain bike from Costco that I got about a year or two ago. The brakes and gears need a tune up and it needs a better saddle but all in all it is a good bike. I used to use it for weekend trips and local trails but now I want to start using it as a commuter but I find it isn't as comfortable as my boyfriends hybrid TREK 7200FX and was considering getting a New one like his that would be more suiting for the commute. Will it be worth it to change all the things with the old one or is it even advisable to ride a mountain bike on the streets for commuting? Or should I just spend the extra cash to get something I know I will ride? Thanks for any advice. :-)

I am not an expert on bikes by an means, but I do know, no matter how many parts you bolt on that bike it will never be the new one you want. You will probably spend as much money replacing componets as it will cost you to upgrade the bike. My advice (for what it's worth) get the new Trek. :D

Thanks for the advice! :)