this one i'm not too sure, but i think the bank will maintain the monthly installments unless you request for adjustments

but i'll give you a scenario:
let's start with a 200k loan (actually loan amount dosn't matter in thsi example), and you have 50k cash

let's assume your financing rate is BLR - 1.6%

and BLR = 6.6%

thus, the effective interest rate = 5.0%
1. REDUCE PRINCIPAL

Current Principal: 200k
Reduce: 50k
New Principal: RM 150k

Interest charged for 1 month based on previous principal balance: 5.0% / 12 * 200,000 = RM833
Interest charged for 1 month based on new principal balance: 5.0% / 12 * 150,000 = RM625

Interest saved: RM 208
2. OPEN UP ANOTHER INVESTMENT

AS1M: dividend = 6.7% (2013)

Dividend earned over 1 month period on a 50k deposit: 6.7* / 12 * 50,000 = RM 279
you earn RM279 in one month if you invest in AS1M, while you only save RM 208 in the same month if you reduce the principal

multiply the RM 71 x tenure period = ??

of course this is tricky, and you haven't even included the compounding dividend effect will increase your earning, while reducing principal will decrease your savings
again, this is just a rough calculations. many variable are ommitted in the calculations. this is just a simplified calculations to see which is more beneficial in terms of ringgit and cents