Personally I don't think it matters. They're basically the same. The DSi obviously has more features, and future games will make use of those features so it might be worth getting.
It's only an extra £50 so if you can afford it for the best DS model then why not?
Get yourself a flashcart. If you don't really know what that is I'll be happy to pm you with details. Amongst other things, take the DS Lite, for example, it allows you to put in a Micro SD card and play music, homebrew games etc. You can get these for the DS lite and DSi at similar prices anyway (under £20).
I got the girlfriend a DS lite at christmas knowing the DSi was coming out and to be honest we haven't been inclined to upgrade. There's a huge catalodue of DS games, a lot are for the casual audience anyway (if not all). Depending on how good the console makes future games is another matter.
You can get the DS lite from under £100 from Play with a free games. Don't forget the 5% RAC offer and Quidco:
http://www.play.com/Games/DS/3-/308963/2-/Promo.html
Keep your eye on Hot UK Deals (if you don't already) because they often get posted.
You can get your
flashcarts from Dealextreme amongst other places. Dealextreme I find are the cheapest though, reliable, but delivery can take up to 2 weeks. Again let me know if you need more info about that.
Edit: p.s. Layton is very good yes. I've had a decent go and the girlfriend has completed it and really enjoyed it.