You should do NEITHER at RD.
To answer your question: there is no "which line gets longer faster" as both get 60+ minute lines immediately (literally). Something like 90% of the people at RD go to either A&E or 7DMT. Everyone walks back and walks into the lines of these two rides. At 9:02 each line will have hundreds of people in them while every other line (space, splash, peter pan) will have 0 people. If you arrive at the park, go through the mickey-reader, and then wait in the middle-back so you can watch the opening show by the time you get to 7DMT the line will be near the teacups! Factor in the FP+ arrivers and you'll be waiting until 10:00 before you get to ride. If you are under the tunnels during the opening show you will have a chance at a short wait. If you can push your way through the crowd to be at the very front next to the rope then you can do A&E or 7DMT with a short wait. But if you are behind 100 people there is no point waiting for one of these rides at RD. If you're not in the first few groups you should avoid them at all costs.
7DMT's line will be 60+ minutes from 9:02 AM - close. A&E will be 60+ minutes from 9:02 - close.
Peter Pan will be <10 minutes until 9:45. The mountains will be <10 minutes until 10:00. Take advantage of short waits at RD to maximize your rides. If you can't be within the first 40 people to 7DMT or the first 20 people to A&E then you'd be wasting precious time when lines are non-existent for all non-A&E/7DMT attractions waiting in a line!
At 1:00PM you will have the choice to wait 90 minutes for Splash Mountain or 90 minutes for A&E. Either way you're waiting 90 minutes mid-day for anything good. So why not wait for A&E then and do Splash with 0 wait at 9:00 AM, then do Thunder and Pirates and Jungle Cruise...all in the time it would've taken you to do A&E at rope drop.
If you want both A&E and 7DMT I recommend doing them at the peak time of 1-5PM. Lines for all attractions will be very long during this time period, so you won't wait much longer for A&E than you would for Haunted Mansion. If A&E is 120 minutes and HM is 60 and Peter Pan is 60 then between 1-3 you can do A&E or HM+Peter Pan. But at 9:00-10:00 AM you can do either A&E with a 60 minute wait or HM+PP+Jungle Cruise+Splash. By avoiding A&E at Rope Drop you get to do 2 more rides that day.
At Rope Drop lines for everything else are shorter than they've ever been so take advantage of it!
In the past at RD some people went to Space, some to Splash/Thunder, some to Peter Pan and all the top-tier rides got 20 minute lines by about 9:30. Today 90% go to A&E and 7DMT so there's nobody at the old headliners until 10:00. The first hour at MK in 2015 has the shortest lines I've ever seen and I've been going consistently since 2009. However, the lines at MK in 2015 between 1-5PM have the longest lines I've ever seen. No matter what you want to do you're going to have to wait an hour. So why not spend the time you're forced to wait in line doing A&E and 7DMT?