The old DS's and DS Lite's were the best way to play single player GBA games . Still worth keeping for that purpose .
The honor of the best way to play single player GBA games belongs to the Game Boy Player.
But you are absolutely correct that the best way to play your GBA games in portable form would be with the DS/DS Lite. At the time, playing GBA games on that amazingly bright and truly backlit DS screen with the built-in stereo speakers was kinda like going from watching your favorite VHS movies on DVD for the first time.
To play all GB/GBC/GBA games in portable form, the second model of the GBA SP with the much better screen is as good of a device as you'll get that isn't the Analogue Pocket.
I have to disagree on this one. The GBA-SP 101 model has it beat. It's not as large or clunky as a DS, and it's screen is fantastic and bright.
I've found that DS screens frankly suck in the sunlight. While the GBA-SP does great in it.
Now the sound, yeah, can't argue that.
But as you said, Game Boy Player is hands down the best way to play GBA. You don't have to weigh the pros or cons like you do DS vs GBA, it's just better in every way.
even an ags 101 sucks in bright sun light. I have multiple ags 101 devices and if where talking at the correct conditions you could literally use a normal gba sp with pretty much the same results. it really isn't that amazing it's only slightly better under brighter conditions than the average sp. even the most modern nintendo devices on the brightest setting suck in bright sunlight while they drain that battery ultra fast aswell. playing handheld games you'd usually rather do in the evening or indoors in wich it kinda defeats the purpose of having a better backlit sure there is a difference but it's not much, for my taste anyway all handheld devices just suck to play outside.
also what ds where you using classic or ds lite? fairly sure the ds lite and other newer ds models like dsi had the better backlit the original was allot less bright not to mention having less options in terms of brightness. in bright sunlight a ds lite would give the exact same results as a gba sp 101.
Pretty comparable to the ags 101 I'd even argue the exact same for a fraction of the cost pretty much no difference at all. the price alone is a reason why you get a ds lite instead of an ags 101. and if you really wanted to have a handheld with a good feel you wouldn't go for an og ags 101 anyway. you would get the classic gameboy advance and give that an backlit screen mod in wich you get the excellent backlit plus it being the most comfortable in them hands.
Modded handheld devices are pricy as hell though. I'm always shocked how much people are willing to spend on modded handheld devices even the game gear or especially the lynx wich don't really have amazing games if I'm being honest.
also if where talking playing games on the ds instead of using the original ds stereo speakers get a cable and connect them to a set of actual speakers, gave me a fresh experience on a ton of games pretty great to play them games on some actual good quality speakers. You can't play ds games on a gameboy player after all handheld exclusive sadly.