if not being allowed due to space I park my mtb and roadie side by side not much more space is taken up. If its because of the Mrs well it comes down to the bottom line and if the bottom line is the same and you have two bikes whats the difference? You should be free to choose a hobby as long as it doesnt take more time or a whole lot more funds plus if you spread time between two bikes they last twice as long.
-if you sell your mtb, will you get something in a roadie you really want? being something "slick" you probably need to by new gear, shoes, knicks, jerseys etc.. the list is never ending you may end up spending more than you planned.
What if you find out it was a phase and you have sold your mtb?
Find a cheapie road theres always someone who has decided they are bitten by the bike bug and buy new bike and discover its not for them who wants to sell. If the speed and miles sound exciting, a roadie may be for you. If you really get into sell your mtb later and use the money for upgrade, gear etc.. (putting slicks on a mtb and riding on the road is not the same)
Only you know what does it for you, everything else are just opinions.
If you can't decide and must decide, make two columns on paper put MTB in one and ROAD on other and write all the pros and cons in each and add it up and decide. If you like in Column 3 put the "Mrs" hehe
** I remember my first time on a real offroad trail. my passion for it was much more than rolling on pavement.