Back to the original question without all the arguing.
I contact print from my 5X7's more than enlarging them and find that they are, to my eye, adequate in size and the quality of a fine contact is hard to beat. I can't put my finger on the difference but when viewed as a contact then as an enlargement oftentimes the enlargement just has something missing.
I'd say go for the 5X7 its easier to haul around, film and holders are cheaper and just about any lens you have for 4X5 will cover the 5X7, excluding tessar types like the xenar, optar etc. An older symar(convertible) of 180/? would be a great starter lens.
Burning and dodging I have not mastered yet with contacts, but I have found that if your negatives aren't too dense(bullet proof) that under the enlarger light you can actually see throught the negative: the white paper underneath reflects the light back so you can tell where you are on the negative. It helps quite a bit to turn off the safelight to visulaize.
I be interested in hearing more tips on this subject(contact burning and dodging) sans the arguments.