Which is better for an older laptop?

Check the slot first. If the laptop has only an M.2 2242 slot, the Transcend 430S is the one that fits. If it has a 2.5-inch bay, the SanDisk 520 gives you more room and faster writes.

The SanDisk 520 at 1 TB holds about twice as much as the 480 GB Transcend 430S.

Reads are close, both around 550 to 560 MB/s. The SanDisk writes faster at 525 MB/s against 450 MB/s for the Transcend.

Does the Transcend's DRAM cache matter?

It helps keep speeds steady during long writes and heavy small-file work. The SanDisk listing does not name a cache, so the Transcend has the edge there.

Is the SanDisk worth stepping up to?

Yes if you have a 2.5-inch bay. You get double the capacity and quicker writes for the same SATA interface.

Neither listing states endurance figures. The Transcend names LDPC coding and a RAID engine for data integrity, but that is not a lifespan rating.

Transcend 430S SSD vs SanDisk 520 SATA SSD

Buy the SanDisk 520 if you want a 1 TB drive for a desktop or a laptop with a standard 2.5-inch bay. Buy the Transcend 430S if your machine only takes a short M.2 2242 SATA card. The SanDisk is the faster writer and holds twice the capacity, and its owners rate it slightly higher. The Transcend is the smaller, lighter drive with a DRAM cache and power-loss protection, which suits tight builds and embedded systems.

Choose the Transcend 430S if your laptop or mini PC has an M.2 2242 slot and no room for a 2.5-inch drive.
Choose the SanDisk 520 if you have a 2.5-inch bay and want 1 TB with quicker writes.

Prices

Transcend 430S SSD: $44.99 at eBay (checked today)
SanDisk 520 SATA SSD: $129.00 at eBay (checked today)
Difference: the Transcend 430S SSD is $84.01 cheaper than the SanDisk 520 SATA SSD today