Hard Drive Storage and RAM
The MacBook Pro comes standard with a SATA hard drive. However, you can choose to upgrade to a Solid State drive during checkout, which will give you more overall speed. The MacBook Pro has a storage capacity of over 750GB in SATA form, and a storage capacity of up to 512GB in solid state form, when configured in the Apple Store.
The MacBook Air comes standard with Flash memory in 64GB, 128GB, and 256GB configurations. Built-in flash memory is much the same as having a solid state drive, and much quicker than standard HDD drives.
The difference here is that Flash memory comes standard with the Air but a solid state drive does not come standard with the Pro. Choosing a solid state drive with the Pro will up the price significantly at checkout.
If you want capacity go with the MacBook Pro. Whether it be SATA drive of solid state drive, there are just more options on the market for expandability, even if you choose to not purchase an upgraded hard drive from Apple.
If you want hard drive speed, go with the MacBook Air. The Flash memory that comes standard is lightning fast, and has been known to boot from power-off in around 10 seconds. However, capacity is very limited.
Do not let RAM memory be a contributing factor in which laptop your purchase. RAM is cheap, and with a bit of research, you can upgrade the RAM in either one of these laptops. Do consider however, that the MacBook Air has a lower RAM maximum (4GB) than the MacBook Pro (8GB).