I just have a regular 3ds but I want the new regular 3ds but it didn't come out here in the US you're so lucky though. The XL never appealed to me much because I like to carry mine around in a small purse I have.
I'd go with an XL since I have one from personal experience, you could either transfer data onto the new one or just put the cartridge in, it doesn't matter the data will still be there. When you don't transfer the data over it will say "recreate the town" which will mean just create the data, not start over. I thought it meant start over until a month later took a gamble and said yes. I face palmed so hard when my town was there and regretted not taking the chance earlier. Other wise it's fine if you switch or not. No regrets now. The XL does have better graphics and holding a 3ds the same size as my old one doesn't feel weird and if you switched for the smaller one it would feel weird for awhile. I even go used to the c-stick after only 5 minutes of playing MH4U so there's another reason for you go get the new 3ds. I have no regrets buying the XL, but buy what ever you want instead of reading my biased opinions.