SD Card vs Ext. Hard Drive

In a nutshell, no.

It is safer for your data to be stored completely separate from the place where it is used. While having your backup self-contained within your laptop is extremely handy, what would happen if your laptop was lost, stolen, burned in a fire, etc... Not only would you lose your original data, you would lose your backup at the same time.

It's best to store your backups separate from your computer. If your data is particularly critical, you would want to have multiple backups and store a copy of your backup in a completely remote location. Why? A thief breaking into your home might steal your laptop AND your external drive with your backup. Or, a fire might burn your house down with your laptop and backup inside.

While an SD card could be stored separate from the laptop, SD cards are more prone to failure than an external HDD or SSD (the same would be true of USB thumb drives). However, you could negate that danger by having more than one SD card, keeping identical backups on each one and storing them in separate places. No matter what backup media you use, you should always have more than one backup, preferably at least one kept onsite and another one kept offsite (and they should be swapped out frequently to keep the offsite backup as up to date as possible).

Since your wife's website is probably unsecured, make sure that you encrypt your data before uploading it.

All hardware devices are subject to failures.
But, as we see almost every day someone with a failure on his external HDD device, I would use a SD for backup. It's all electronic, no moving parts.
The major problem to external HDD is that they are very fragile devices.
- Never put the HDD on the vertical. One fall to the horizontal and it's gone forever.
- Once it's running, don't touch it, don't move it.