R22 is right. Frida had the better range, greater technical mastery and musical ability, but her lower tessitura and soft, mellow voice don't lend themselves well to most pop songs, which require a sharper, brighter tone and the ability to consistently belt relatively high - two things that she lacks. Agnetha's tone is exceptionally beautiful and it has a peculiar intensity that gets people's attention, and she could belt quite high when she had a good day (which wasn't that often). Anyway, ABBA songs rarely required the main vocal line to go above E5, and most of the higher notes were harmonized in head voice. If you think about it, nowadays most singers constantly hit F5 and above, so by more contemporary standards their songs weren't very high.
I am not a musical expert, but from what I've been able to gather, Frida's range is B2 - E6, but she was never comfortable belting above B4. Incidentally, the need to cover the C5 - E5 range in full voice is what gave her harmonies that very metallic, slightly reedy quality that was essential to the ABBA sound. As R22 shows, her head voice was gorgeous and she had absolute control over it. In her live performance of En Ledig Dag she goes up to E6 like it's nothing.
Agnetha's range is D3 - D6, but she once screamed an Eb6 during a live performance of the King Kong Song. Her tessitura is higher (she is comfortable belting up to E5, but could very occasionally go up to B5). Her tone around G4 and upwards was very crystalline but remained warm, which was also essential to achieve the ABBA sound. Without either girl, the harmonies wouldn't have been quite as beautiful and the songs wouldn't have had so much of an impact.
The interesting thing is that both voices compliment each other extremely well in every area - Frida's was more powerful up to G4, and it had a velvety, dark quality to it that was complimented by Agnetha's creamy lower and middle range (think about how Agnetha harmonizes below Frida in Fernando, for example). Above G4, Agnetha's voice became stronger and brighter, whereas Frida developed a girlie, metallic quality that greatly complimented Agnetha's own brightness. Together and solo in the context of ABBA, they both shone like very few singers can do it.
Their solo material is very uneven, though. The repetitive Something's Goin On is much better than the very generic Can't Shake Loose, partly due to Phil Collins' great production. The rest of the album is a bit lackluster, although R12 is right and Threnody and To Turn the Stone are gorgeous... Sadly, the rest of songs are rather mediocre and bland. I have always hated Shine and its clunky production, not to mention that Frida's voice isn't well suited to dance music at all. She sounded completely lost on that record, to be honest. As for Djupa Andetag, there are some really interesting numbers in there, but the production is very monotonous and there is a lot of boring filler.
In overall, Agnetha's discography is better and more varied (I Stand Alone is a great album), but her more conventional song choices and production didn't make her stand out enough. Also, My Colouring Book was insanely boring (did we need yet ANOTHER album of standards and hits from the 60s?) and A is a very pretty and enjoyable record, although maybe a bit too ballad heavy. As for her Swedish records, I have never, ever been able to like them - in them, she seems to be competing for the first prize in the Most Boring Music in History competition.
Sorry for the lengthy explanations, but this subject is quite enjoyable.