To use the Opera browser (which does full HTML rendering including pictures) you need a slot2 GBA expansion cart because Opera requires extra memory to run. Both the EZ 3in1 and eWin 2in1 are good choices, but there are some differences. I prefer the eWin 2in1 because it's simpler, cheaper, and has more memory available for the browser. Both will also allow you to play GBA games that are stored on your slot1 cart.
Even with memory expansion, you'll find that the Opera browser is quite slow if you have images turned on. You can use other browsers like the one in DSOrganize for text-only browsing (or turn off the display of images in the Opera browser).
There are other differences between the eWin 2in1 and the EZ 3in1. The EZ 3in1 has rumble built-in. If you've never used rumble, it's not really as exciting as it sounds (it eats up the battery, it's noisy, it's not particularly "rumbly," it only works with a handful of NDS games and no GBA games, and sometimes it gets "stuck" rumbling during games that don't have rumble support). Plus you can buy a better eWin rumble pack as a separate cart for around $4, so I definitely wouldn't consider the rumble as a major factor deciding between the two.
Other differences pertain more to playing GBA games. Both the EZ 3in1 and the eWin 2in1 have enough internal memory to run a single GBA game that is stored on the microSD card of the slot1 cart. The EZ 3in1 has two types of memory that can be used to run a GBA game: NOR flash memory and faster volatile memory. The NOR memory of the EZ 3in1 can be used to store a game which can then be run multiple times even after you've turned off your NDS (since it's stored in the non-volatile flash memory). But it can take a minute or two to store a game to the NOR memory because it's very slow at saving (once it's stored, the game will run fairly quickly). Since it only takes a few seconds for a GBA game to load into the faster volatile memory, I'm not really sure there's much advantage to waiting for 1 or 2 minutes to store a game to NOR memory. Unfortunately, the EZ 3in1 doesn't have enough of the faster volatile memory for larger GBA games, so you have to use the slower NOR memory for larger games.
The eWin has enough of the fast volatile memory to store the larger GBA games. Because it has no extra NOR memory, it's easier to use and it's also much cheaper.
-Bri
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