"Dynamically update DNS A and PTR records for DHCP clients that do not request updates" is more for legacy clients such as NT4 or clients that do not support registering there DNS through DHCPMy explanation:
Discard can help clean up DNS so you don't have stale records on your network once a client changes IPs or moves to another subnet.My explanation:
Of course if you have no need to register DHCP clients in DNS, as they are most often client computers, you can disable these optionsMy Explanation:
Dynamically update DNS A and PTR records for DHCP clients that do not request updates (for example, clients running Windows NT 4.0) - This option can be selected if you have Down level clients (95/98/NT) or third party OS who doesn't have functionality of DDNS (Dynamic DNS Update). If we uncheck them, mentioned clients will unable to register themselves with DNS.
Always dynamically update DNS A and PTR records" - Which mean we are asking DHCP to register DNS records on behalf of client machines. If you run DHCP on DC, DHCP will not register records in DNS unless we set credentials (standard user credentials). You can create one user and use his credentials for DNS registration, you don't need to use Admin accounts.credential tab will be found under IPv4 properties\Advanced tab
Instead of above option you can use another option "Dynamically update A and PTR records only if requested by DHCP client machines". If we select this option, client will register A records and DHCP will register PTR records. We need to set credentials for registering PTR records.
Discard A and PTR records when lease is deleted
Please understand that discard does not mean delete records by DHCP server itself immediately, system will clean these records only if DNS aging/scavenging settings on DNS server is set properly.
You must create a dedicated user account and configure the DHCP servers with its credentials under the following circumstances:
•The DHCP server is configured to perform DNS dynamic updates on behalf of DHCP clients.
• A domain controller is configured to function as a DHCP server. Without the dedicated user account, secure updates will not work.
• The DNS zones to be updated by the DHCP server are configured to allow only secure dynamic updates.
Check mine comments in below posts on same topic for more information regarding DNS scavenging
Since you are running Windows 2008 R2, also check mine comment in below article for more information regarding DNS proxy Updates and DHCP name protection.
Lastly check below TechNet blog