Which is better for gaming?

The C5. Its 144Hz panel and VRR handle higher frame rates than the C1's 120Hz. Both are G-Sync compatible and have low input lag.

Is the C1 worth stepping up to for the bigger screen?

If you sit far from the TV, yes. The 65-inch C1 gives a noticeably larger picture. If you sit close or game at high frame rates, the C5 is the better set.

Which has better HDR?

The C5 supports HDR10+ as well as Dolby Vision, so it covers more sources. The C1 handles Dolby Vision IQ and HDR10 but not HDR10+.

Neither has a clear reliability edge; both are OLED panels with similar burn-in risk. The C5 will get software updates longer because it is newer.

Do both have four HDMI ports?

Yes. Both list four HDMI inputs, plus Ethernet and Wi-Fi. The C5 adds a CI slot.

The C5 uses a newer OLED panel generation and is generally brighter than the C1. Neither matches a bright-room LCD, so avoid direct sunlight on either.

LG OLED C5 Series 4K TV vs LG C1 OLED 4K Smart TV

Both are LG OLEDs with 4K, four HDMI ports and Dolby Vision, so the picture quality gap is small. The C5 is the newer set: 144Hz, a brighter panel generation, HDR10+ support and a newer webOS. The C1 is a 2021 model at 65 inches in this listing, so it wins on screen size for the same room. Buy the C5 for gaming and the latest processing; buy the C1 if you want the bigger screen and can live with 120Hz and an older processor.

Choose the LG C5 if you game at high frame rates, want HDR10+ alongside Dolby Vision, and prefer the newest panel and software.
Choose the LG C1 if you want a 65-inch OLED and 120Hz is enough for your consoles and streaming.

Prices

LG OLED C5 Series 4K TV: $895.99 at eBay (last seen Nov 9)
LG C1 OLED 4K Smart TV: $1,299.00 at Amazon (last seen Nov 9)
Difference: the LG OLED C5 Series 4K TV is $403.01 cheaper than the LG C1 OLED 4K Smart TV at the latest prices (Nov 9)