Hi willow,
Welcome to gear-monkey! I have experience with the amps you speak of and I can tell you this.... you can't go wrong with either.
The flextone (line 6) is a very versatile amp and sounds good... It's a cheap gigging amp and it is versatile. It's digital and one of line 6's lower end models so you can hear and feel it. Your trained ear will be able to tell the difference from valve amps. But for the price, it's a good bargain! esp. since it has all the built in effects. (make sure you get a FBV shortboard) -- you could probably find the complete combo package for around $500ish
Personally, I would look into the Line 6 Vetta instead, that is MUCH more amp! And they sound absolutely amazing!
Now for the Mesa Boogie Roadster.... The Roadster is the almighty amp over the Line 6- It's valve =D Very versatile with 4 channels. Unlike most valve amps, it has independent reverb and each channel is switchable to 50 watts or 100 watts and switchable to 6L6 or EL34s.
The amp is another great gigging amp, but much more expensive.. so you will probably pamper it much more. I would buy a second hand Boogie Roadster.. which will run about $1700 - $1750ish for a nice head. (the 2x12 combo Roadster are heavy to lug around. Therefore, I am turned off)
You might like the EL34s in the head, they're hotter valves and will break up better for the AC/DC -- classic rock tone. However, the 6L6GCs are my favorite (I play metal and love the thick tone)
Here is a blog that I wrote about the Roadster:
http://74.52.142.82/~cbeers/gs_blog....ofile=1&bid=23
I'd go with the head and 2x12 Recto cab with Celestion Vintage 30s if you choose the Mesa Boogie Roadster. It's easier to lug around over the Roadster combo and sounds HUGE! If I were to decide for you, I'd pick the Roadster over the Line 6 Vetta. (I wouldn't consider the Line 6 Flextone as I doubt it much of an upgrade)
It really depends on your budget.... I'm sure you would be happy with either!