Yes, the Evo and Pro are pretty different drives, as their price difference reflects. Hence I'd rather go with the vanilla 840 than the Pro, if the Evo is not yet available (in Germany shops expect to get them within 1 to 2 weeks). The benefits of the Pro are higher performance and longer write endurance. Neither of which matter for typical desktop / laptop use: the performance difference should be unnoticable without a stop watch, and depending on your laptop you might already be CPU-bound with slower SSDs. Or you might be limited to SATA2, which would make both drives perform similar. And write endurance: it's absolutely safe at regular desktop workloads, and even more so at 500 GB capacity! If you can afford the Pro it's surely a good drive.. but I don't think it makes any sense to spend that money over a vanilla / Evo, even if you have it.
Don't bother with partitioning unless you want a D: drive anyway. Just never fill the one partition / the drive completely.. I'd say leave at least 10 GB free. Performance will be somewhat reduced compared to an empty drive, but you're not suddenly starting to write 10's of GB of very small files randomly to the drive, are you?
Regarding the capacity choice: I don't think it makes all that much sense to store large videos on SSDs either. I'd rather archive them on an external 2.5" HDD, which could easily be taken along with the laptop. For the price difference between a 250 and a 500 GB SSD you could easily afford another external 1+ TB HDD, in addition to the 500 GB you'll already have after the upgrade.