I'm tiling the concrete floor in a small area in the basement where two slabs butt up.

Should I put tile over the seam, splitting the tile along it, or run the tile along the seam so the grout line will be over the butt?

If you can lay the tiles factory edge right beside the crack / seam so the grout line on top of the seam that's they way to go.
This way if the two slab move it should only crack the grout line. Say it took 3 or 5 years for this to happen its so much easier to re grout than to chip up tile and try to match the old tile not to mention not enough left over from the original job. Two theories here
1 Tile at butt like I said
2. Tile over the butt and if the conc. Butt moved a small amount the floor hopefully wouldn't telegraph the crack through the thinset and tile.

In Puerto Rico. I do construction work there every year. The houses are all concrete and block with concrete roofs. Most all house floors are tile on concrete. Puerto Rico gets very small earth tremors. So to keep there tile floors form showing cracks they mix up by hand and spread 1 1/2 of Portland cement on existing concrete floor and set tile on the 1 1/2 mortar bed. There are two reasons for this. One is the floor is raised 1 1/2 plus tile and this height acts like a door threshold for rain to stay out. To the 1 1/2 bed helps to absorb some hair line cracking in the original concrete floor and hopefully the mortar bed won't transfer that to the top of the tile. Bascially an 1 1/2 floating mortar bed so to speak ( trivia in tile here in the states is little known)

Signed 21 boat

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I agree but the tiles at this seam and let the grout cover it
Good luck. I know it is a hard job . We tiled two floors one in the bathroom twice because we had no idea in the world what we were doing!