It really depends on how much of a 'hands-off' experience you want. Ritchie Bros. is the gold standard if you want that live auction energy and a massive gloabl buyer base, but you do have to deal with the logistics of getting your gear to their site.
IronPlanet is great because the equipment stays on your lot, which saves a ton on transport costs. That said, since they are both part of the same parent company now (RB Global), you're getting similar reach either way. If you're looking for a middle ground with more modern tech, Boom and Bucket or Marketplace-E for more control over the final price. Personally, I'd stick with Ritchie Bros. for the pure speed of the sale.