I've done both. I much prefer working 3 days together, and would strongly recommend Tues-Thurs if you can do it.
When I went back when DS was almost 1 I did Tues, Weds, Fri as I thought it might be easier to manage my workload if I was only ever out for one working day. However, I found that it didn't make much difference to people when I was around, and actually Fridays were very quiet as many part-timers have that off or people work from home or people do long weekends and have it off. And for me, it was weird finishing on Weds but not having finished my working week. I also often missed some important Thursday meetings or had to change my days, because with lots of people not being around on Fridays, big meetings were often scheduled on other days of the week.
So I spoke with my manager and switched to Tues, Weds, Thurs. I rarely notice an issue with being out Mon and Fri - they are effectively two consecutive work days but people don't really think about it like that as it straddles the weekend, so my work availability hasn't really suffered and work is fine as people still only think of me as being out one working day at a time - whereas if I worked Mon-Weds or Weds-Fri I think it would cause more problems as people wouldn't be able to get hold of me for 2 days in a row. And for me it is great as I get a 4 day weekend and plenty of time to recharge my batteries.
For DS I don't think it makes a huge difference. I was worried he would find being away from me 3 days in a row hard, but I don't think he minds either way. He had a nanny for the first 10 months I was back so I wasn't worried about him being too knackered at nursery which would definitely have been a consideration otherwise. He started nursery in January (aged 21 months) and goes 3 consecutive days and he is fine - tired by Thursday evening but copes just fine, and then has 4 days off. Plus I think it helps him not to be chopping and changing so he has a run of nursery, then a run of being at home rather than never quite knowing what is going on each day.
Tues-Thurs is also brilliant because I effectively end up with more holiday - you only get proportional bank holidays as a part timer, so I get 5, but as they rarely fall on my working days (only Christmas, Boxing Day and New Years Day could) then I get to take my allocation off in lieu at other times so I can effectively treat them as holiday on top of my normal holiday allowance. If I worked on Mondays then if there were more than my 5 on Mondays (4 always are Mondays and the Christmas ones could be), I'd have to use holiday just to cover the extra bank holidays IYSWIM.
For our home life, Tues-Thurs really helps too. I don't get that Sunday evening feeling until Monday so it means there's only ever one of us moping around at a time! And it also means that anything we didn't get done over the weekend I can do on Monday. So Mondays is often my day to do boring stuff like supermarket or errands, then I get Friday free with DS and can get things in shape for the weekend if there are any odd things that need doing (e.g. make sure we have eggs, bacon and milk in for a Saturday fry up!) so as a family we have slightly more time at the weekend to do fun stuff rather than having to do all chores etc.
Anyway, personal preferences and all, but I have to say I find Tues-Thurs great and don't really want to work Mondays or Fridays ever again if I can possibly help it!