Depends on the version:
The early one that came with the original J class does not have pickups. The later Spectrum Excursion tender was simply that tender with a fresh paint job.
If you get the later Spectrum "Aux Water Tender" (89951 is the part number on the undec in my hand) it has pickups and standard split axle tender wheels that come with all the Spectrum tenders.
Looks like there are some variations. I have a Spectrum set which has the #611 loco and the aux. tender. The aux. tender's trucks have 1-piece axles and no electric pickup. Also the loco tender and aux. tender use different type trucks. The loco tender's trucks (whatever that type is called) do have electric pickup.
As far as size goes Scott, The Bachmann and Kato (GS4) trucks are identical. Kato truck is not larger (it shouldn't be as it depicts the same truck). I just now compared them. And yes, Kato trucks have the typical excellent axle-end pickup built-in. I would recommend the GS4 trucks too. I bought a bunch of them when they were available from Kato, but you migth find them on a secondary market.