Rule of thumb: under 3 hours, I’d take the bus if the saving is meaningful. Around 3-5 hours, I’d take the bus if it’s direct, daytime, and the station is convenient. Over 5 hours or overnight, I’d start valuing the train much more highly unless the price difference is huge.

In Spain, buses are a very normal and often solid option. Alsa is the big national one, and their Comfort/Supra-type services can include things like WC, Wi-Fi, entertainment, footrests and USB depending on route. Avanza also has better-equipped Multimedia/Plus/VIP coaches on some routes. If Andalucía is on your route, buses can be especially useful: so much of the charm sits in hilltop white towns, olive country and old rail corridors turned into greenways, not just the high-speed city pairs.

In France, the long-distance market is mostly FlixBus and BlaBlaCar Bus; both advertise reclining seats, power/USB and toilets, but I’d expect them to feel more variable than trains. That said, a road leg in southern France can have its moments. If your route passes near Occitanie, the Millau Viaduct is one of those bits of infrastructure where the window seat actually earns its keep.

Also, as someone already said, I wouldn’t count on Wi-Fi being perfect, and I’d keep essentials like passport, wallet and electronics with me for convenience rather than putting them in the luggage hold.

If you want a quick side-by-side view, Omio can be useful for comparing bus and train times/prices in one place, then you can book there or directly with the operator.

For a shoestring trip, I’d save money on the easy bus legs and spend on trains where comfort will protect the rest of your day.