In the free Lamentations of the Flame Princess Referee Book, there's a section (p. 70-71) which talks about comparing the Specialist class to the Thief class in old-school D&D and similar games. A table is provided which demonstrates how to convert the Thief's percentile-based skill spread to the Specialist's d6-based skills. I thought it might be interesting to compare how many skill points the Thief's proposed skill spread gives a character based on this chart, in comparison to the number of skill points a regular Specialist gets. Remember that skills in LotFP start at 1 in 6 for all characters, and they max out at 6 in 6, so it takes 5 skill points to raise a skill from the minimum score to the maximum.
Level 1 Specialist 4 (Total 4) Thief 7 (Total 7)
Level 2 Specialist 2 (Total 6) Thief 0 (Total 7)
Level 3 Specialist 2 (Total 8) Thief 1 (Total 8)
Level 4 Specialist 2 (Total 10) Thief 1 (Total 9)
Level 5 Specialist 2 (Total 12) Thief 1 (Total 10)
Level 6 Specialist 2 (Total 14) Thief 4 (Total 14)
Level 7 Specialist 2 (Total 16) Thief 2 (Total 16)
Level 8 Specialist 2 (Total 18) Thief 0 (Total 16)
Level 9 Specialist 2 (Total 20) Thief 1 (Total 17)
Level 10 Specialist 2 (Total 22) Thief 3 (Total 20)
Level 11 Specialist 2 (Total 24) Thief 2 (Total 22)
It looks like the classic Thief gets an early lead, but is quickly eclipsed by the Specialist and never surpasses it again.
On the one hand, if it's true that most LotFP players are almost never going to get to level up their characters, as I've seen some people say, then you could argue that the Thief is actually the better class simply on account of it having higher skills at level one, the one true level that you're going to be stuck at anyway.
On the other hand, the Specialist clearly seems better to me for two reasons. First, the Specialist gets to distribute those points as the player sees fit, allowing for different character "builds" (a term which probably makes some old-school players cringe, but I feel like the shoe fits here). That way, you're not stuck playing a Thief, but instead you could be a translator or a scholar or a wilderness survival expert or a pirate or whatever, and that's super cool. Second, if you do manage to level up, you're going to get way more points in the long run as a Specialist, and it won't even take that many levels to outclass the Thief. It's probably also worth mentioning that the Thief doesn't get any skill points at level 2 or 8, which is boring, and the table only goes up to level 11 in the Referee book, so I don't know if the Thief maxes out at 22 points or not - I guess it would depend on which game's Thief you're converting to the d6 skill system.
So yeah, I prefer the Specialist. It's definitely one of the nicest things LotFP brings to the B/X formula, at least insofar as I've experienced it.