Short answer: you will be fine either way, but I'd take the large SSD.
Longer answer:
Speed
The bigger variants of the same model usually have better read and (especially) write speeds. Now, depending on the performance/number/kind of your SATA-controller on your mainboard, it might be faster to have 2 SSDs working in parallel. Or you create a hardware-RAID0 out of the 2 SSDs which might increase speed a little.[1]
Whatever you do, most current SSDs will be fast enough[2]. While differences can be measured in benchmarks, but you should not notice a much difference in day-to-day use.
-> might be no difference in real life for desktop PCs
Durability
Modern SSDs have spare blocks that can be used in their wear-leveling algorithms to improve the durability of the drive. The amount of spare blocks is often proportional to the size of the SSD.
What that means to you:
- If you have 2 drives and your write-operations are uneven (much more writes on one of the drives), then this drive might die earlier. But your other drive will be unaffected.
- If you use a big drive, then it will have more spare blocks to distribute the writes to and it will survive longer. But IF it fails, all your data is affected.
Since you want to use the SSD mostly for gaming and apps, most modern SSDs will be durable enough for you. And you should have a backup of your important data anyway, so...
-> might be no difference in real life for desktop PCs
Future value
Say, in 2 years you want to buy a new SSD (since even the big one is now too small to hold all your games). If the old one was bigger, then it has better value for you, as you could use it to upgrade an old notebook that might have a smaller SSD (many notebooks can have only 1 SSD, so only one of the 2 small ones could not be used otherwise).
-> In this category, the big SSD wins outright.
[1] However, if one of the SSDs breaks for some reason, all data will be lost. Statistically, this is more likely to happen for the RAID0 than for a single drive. Also, the RAID controller might break.
[2] Compared to a regular HDD