You are kind of asking us to compare an apple to an orange. They are completely different.
You can read up more on NPP here ->
http://www.evolutionary.org/nandrolone-phenylpropionate-npp/
You can read up on testosterone more here ->
http://www.evolutionary.org/testosterone
Rather then pick one or the other, the best option would be to use BOTH. It is very to experience libido problems when using deca/npp by itself. That's where the nick name 'deca dick; comes from.
Nothing will put on size and strength like the old school classic bulking cycle of test + deca/npp + dbol ->
http://www.evolutionary.org/testosterone-deca-durabolin-dbol-cycle The dbol isn't essential but it would be a good addition if you are trying to bulk.
Both deca/npp and test are known to cause a lot of water retention. Especially if you aren't using the proper support supplements with your cycle.
Remember to include the proper support supplements with your cycle:
1. An AI to keep estrogen in the low-normal range. Aromasin at 10mg EOD is a good dosage.
2. An anti-prolactin drug to keep prolactin in the low-normal range. Cabergoline at 0.25mg EOD is a good dosage.
The support supplements are very important because high estrogen and prolactin levels can lead to side effects like gyno and water retention.
I would also add in a good liver aid supplement like n2guard from n2bm.com when using the dbol because it is a liver toxic oral steroid.
Here's what I recommend:
Weeks 1 - 16
testosterone 500mg/week
deca 300mg/week
OR NPP 100mg MWF
aromasin 12.5mg every other day
cabergoline 0.25mg every other day
Weeks 1 - 6
dbol 30mg/day
n2guard 7 capsules/day
After your cycle run the perfect PCT protocol:
http://www.evolutionary.org/the-perfect-post-cycle-therapy-pct/