Say you have to make an 11th level character. which of these do you think is better; Arcane Trickster Rogue 11 or Swashbuckler Rogue 5/Lore Bard 6? Which seems better in terms of damage, spellcasting ability, versatility or just downright coolness?

If you multiclass, you would be missing out on an ASI and reliable talent from being a pure rogue.

Hard to say which one might be best with damage and such, but the multiclass would definitely be better at spellcasting.

Lots of viable options. Is this for a one shot or do you expect to gain additional levels later on? 

AT is really solid on its own. I don't think you can go wrong there.

For a swashbuckler/lore bard multiclass I like either:

• Rogue 3 / Bard 8: Rogue doesn't get extra attack at 5, so I don't think it is as important as other martial classes. Compared to R5/B6, you are trading a single d6 sneak attack for some great 4th level spells like Greater Invisibility, Dimension Door, and Polymorph. Swashbuckler can already help you dip away from danger, so Uncanny Dodge isn't as important (plus you have cutting words for protection).
• Rogue 6  / Bard 5: This is a great skill monkey build, netting you expertise in 6 different skills. You still get the level 5 bard spike, which I think is pretty important for bards, nabbing them short rest inspirations and level 3 spells. 

This is for the 'Against the Giants' adventure in the Tales of the Yawning Portal book, so it will run for a few sessions, and we'll level up maybe twice, and perhaps take the characters on to new adventures.