When I had dc1, I went back FT, as dh was studying at the time so my salary was all needed.
When I had dc2, I went to 3 days a week. Best thing ever.
You have a good part of the week, at work, and keep up with your work skills / qualifications / network / experience / changes in your industry, as well as being "you" for those hours every week, and keeping your brain engaged, having adult conversations, and of course, paying into your pension.
OTOH, you have 4 days a week with your little one, which is fab.
I would always suggest full days - particularly when they are babies (there is an argument for being able to collect them from school when they are older, if you can really pick your hours and you don't have a long commute).
Full days outside of work mean you are not clock watching that time - it gives you time to go out places. Working 1/2 days takes a LOT more than 1/2 a day, as you have to commute and you have to still get the dc ready to take to Nursery CM, etc, then get them there, then collect them etc.
Depends on your contract, you need to think about the fact that 4 of the 8 Bank Holidays are always Mondays. (1 is a Friday, and the other 3 move). If you work Mondays, you usually can't work if it is a BH, but they have to take from your Annual Leave to make up the hours. If you don't work on a Monday, they still give you the hours (pro rata) which is added on to your AL. Not all contracts are the same, but that works for a lot of people, so worth knowing.
Then, depends on your job.
Some people like to spread their days, so they are never too long responding to work queries, etc - so might work Tues, Wed and Fri or something. Others prefer a block - might want to go and stay with Grandparents or a friend for a few days etc. Some people have days when it is better to be in (maybe Mondays are a day when all the stuff' happens in your job) or maybe Thursdays are the day you have your Team meetings, etc.