Yeah, I mean about 1.3, (if 3x, then I'll say triple stronger).
Actually I'm too lazy to calculate it, but to make it clearly:
SR Card Lv Up:
Shimizu Saki, Sudou Maasa (Second), and Nakanishi Kana +9
Yajima Maimi (Hina Festival), Nakajima Saki (Valentine), and Miyamoto Karin +7
Others +8
SSR Card Lv Up:
Takeuchi Akari +12 (I don't know why they make her too strong)
Tsugunaga Momoko (Berryz Koubou Graduation) +11
Miyazaki Yuka (Hina Festival) +9
Other +10
For now just ignore the fact that Maimi and Yuka is weakest compare to other cards (no wonder they're free prizes), now just focus on regular card, a SR card often start with 200 Charm (Max Lv50 without limit break), and a SSR card often start with 300 Charm (Max Lv60 without limit break), then:
SR Card Lv50: 8*49=392+200=592
SSR Card Lv60: 10*59=590+300=890
See? Err...it's 1.5 stronger, not 1.3, sorry.
About the Skill Lv Up harder when Lv higher, I mean Level of the skill, not level of the card, but I suggest you Skill Up max your card before train it in live. It's only cost about 2500 Haro to reach Lv10 (play Easy Mode until you get 10 R cards of that member's group, use all 10 cards in a single training, it'll take 3 or 4 times traning like that). The cost will add 100 Haro for each Lv Up (SR Card start with 500 Haro, at Lv30 it cost 3400 Haro), from Lv31-Lv39 the cost will add 500 Haro and after that it add 1000 Haro for each Lv Up.
Though the cost not really much but I'm just a skinflint so I hate to pay even one extra Haro.