Can't say anything about preferences, but I have or have had a few of those different lines. Here are the ones I've had, and my thoughts on them. Hope it helps.
I have a Backline 112, and think it's a great little combo. I wish it had the 4 band GK EQ, but it is perfect for my needs and expectations of it. For me, it is a small practice combo I can use without disturbing the wife/baby, as well as use the headphone out when I want to be totally silent.
I used to have the Backline 600 head, which is essentially a Chinese made 400RB (which RAG had meticulously monitored for quality) with a distortion channel. Not a single negative to be said about that head from me. I traded "up" to an actual 400RB.
The 400RB is my lower volume back up should my 700RB ever go down. It has done everything I need from it. I could likely just use this for most things, but I like the extra headroom and other options on the 700RB.
The 700RB is my main head, and a fantastic one. Never had a single issue, and can level the rest of my band should I need it to. The RB series is all that you hear, and their reputation is well earned.
The last one is the MB200. Also a great little head. It is my emergency back up and headphone amp. I keep it in the rack with my 700 should I ever have a problem at a gig (or have a gig!) and need the backup right away. Also amazing at what it does. It used to be on my pedal board when I lived in an apartment and wanted to have a practice option without cans, and it was a perfect solution for that.
If I had to comment on the differences between Backline and MB series, I would say they are both fantastic, and can get great sounds. The MB seems a little more hi-fi, and the Backline seems to be smaller RB type stuff. I actually think the OD on my Backline combo is pretty darn great. So if you need OD, go that route. If you want ultra clean (or use pedals for dirt like me), go MB. You really can't go wrong, as long as you make sure your volume needs are met.
If I were you, I would go MB combo and a dirt pedal (if you need dirt). Since weight seems to be your big issue, this would solve that.
Thanks. That'll give me a lot to think about. I should be making a decision tomorrow. BTW, volume isn't an issue; this is intended to gig only at smaller venues (small bars, pizza joints, etc.) I have a 400RB for bigger things and if I need bigger than that, well, I'll worry about that when the time comes. I usually play Thursday nights at a country jam (and last night was no exception) that has a house Fender Rumble that I plug in to. It's nice but it's just not the same.