I'm not a native English speaker, but suppose that "spot the difference" in English is the same as "machigai-sagashi (a mistake-searching game)" in Japanese. Let's give it a try!
The top photo is trainset 51001 of the EMU Tobu 50000 series (hereafter 51001F). It's a prototype of the 50000 series, so, only one set of this type of train is operated on the track. On the other hand, the photo below is trainset 51002 (hereafter 51002F), which is one of the mass-produced sets of the 50000 series. Can you spot the difference between 51001F and 51002F?
The correct answer is listed below. Firstly, 51001F (a prototype) doesn't have a gangway; while, 51002F (one of the mass-produced sets) has a gangway on the left-hand side of the front. Secondly, positions of the front lights are different. They're arranged at the right and left lower ends of 51001F. On the other hand, the positions of the front lights on 51002F are higher than those of 51001F. I'm happy to perceive this kind of fine differences among trains as a trainspotter.
Incidentally, the EMU Tobu 50000 series is a direct current commuter train commissioned in 2005. It's known as Tobu's first aluminum body train. One set is composed of five motorcars and five trailers (5M5T). The electric control system is an IGBT-VVVF (insulated gate bipolar transistor-variable frequency drive) with a regenerative braking system. A total of 9 sets, 90 cars, were built by Hitachi. Their brothers, namely the 50050, 50070 and 50090 series, are also operated on the Tobu lines.
