If you want full compatibility while using original GD-ROMs at the expense of video quality, it will probably have to be the RT2X with S-Video, plus an active HDMI to VGA converter (like those by Portta or Tendak) to connect it to the OSSC. You also have the option of using the RT2X directly with your TV, but you’re limited to line-doubling (need the OSSC for 3x/4x/5x).
If you’re willing to forego original GD-ROMs, you can swap out your GD drive with an optical drive emulator, like the GDEMU (Which you should buy directly from its creator, instead of one of the Chinese knockoffs), which will let you force VGA for titles that don’t normally support it, and it will let you run modified disc images for that handful of games that don’t support RGB. It also supports a button combination to reset the system for changing games without having to get up. A GDEMU (or USB-GDROM, really) combined with something like the BeharBros Toro (Or even the HDMI mod below) should get you full compatibility and at 480p.
There’s also an HDMI modification in the works for the Dreamcast, which will provide HDMI output directly from the console, can be used with either original GD-ROMs or an ODE, and does not interfere with the original analogue output.
I’m not sure how it handles that handful of games that don’t support RGB; I imagine it will digitize and optionally line-double them just as it will with the RGB-but-not-VGA-compatible games, but, if it somehow doesn’t, you’d still have the option of using S-Video into an RT2X. Games that don’t normally support RGB are output the same way, so the DreamcastHDMI has full compatibility.