See - the problem with this question is, what is it really asking? If the question is (for any two albums, songs, bands, fruit, whatever), "which do YOU like better?" then there's no wrong answer. Everyone is entitled to their own opinion, and, so the answers are entirely subjective.
Now, if the question is, "which is BETTER (musically, lyrically, thematically)?" then we have a question that can be answered objectively.
So, based on this theoretical perspective, one can certainly claim that they like GR better than SL. No one can deny this or argue or say it's stupid. It's the person's opinion.
However, as to which album is better, there is no room for comparison. SL absolutely destroys GR at all levels. If you disagree, then I wonder if you have actually listened to SL.
Simply stated, SL does not have a single weak track. Every one is amazing in its own right, but none of them stand out as being the BEST track on the album. The album's concept and lyrics flow seemlessly from song to song.
GR is a great album too, but a lot is missing. GR HAS weak tracks. Atonement, while enjoyable, is an unfortunately boring song. The Grand Conjuration has been panned mercilously on this board.
Try a song-by-song comparison.
1. The Moor vs. Ghost of Perdition
Close, but the Moor wins by a nose.
2. Godhead's Lament vs. The Baying of The Hounds
Again, close, but Godhead wins with better clean vocals
3. Benighted vs. Hours of Wealth/Isolation Years
This is too close to call. I personally like Benighted, but a lot of people here seem to love Isolation Years. I also really like Hours of Wealth. Give it to GR.
4. Moonlapse Vertigo vs. Beneath the Mire
No question. Moonlapse slays.
5. Face of Melinda vs. Atonement
Again - Melinda by two touchdowns.
6. Serenity Painted Death vs. Reverie/Harlequin Forest
Some might pick Harlequin forest, but Serenity Painted Death is an absolute classic Opeth song. Your pick. I'm going with Serenity.
7. White Cluster vs. The Grand Conjuration
White Cluster is a fitting end to an amazing album. The Grand Conjuration, while enjoyable, is repetative and boring.
Now - many of this is my opinion, but come on people - look at the facts! GR has no place being compared to SL. The only Opeth album that comes close is MAYH.
Now go forth and spread the word!